I have not read SS's book. These reflections are based solely on what I (quickly) found online -- reviews, synopses, interviews, youtube.
First, what would be my own conclusion: It is of no relevance whatsoever to the underlying question -- which, I take it, has something to do with whether those people who are of [immediate] Jewish origin today ought to be able to emigrate to and live in Israel as a Jewish state -- whether contemporary Jewry's remote ancestors were or were not (a) of a genetically homogenous origin or (b) all or mostly descended from people who were forcibly exiled from Israel.
The only relevant factor is that there existed (and exist) on this planet, a "community of fate" (not necessarily or relevantly of "faith," nor of "race") that suffered severe discrimination and recurrent bouts of brutal persecution in their status as vulnerable local minorities dispersed throughout Christian Europe and, to perhaps a lesser degree, also throughout the Muslim world, for at least the last millennium and a half. Pogroms and pogrom-like assaults have been the relentless sign-posts of this distributed, global vulnerability to discrimination, and the Shoah or Holocaust was its most recent and monstrous manifestation.
Jews (defining themselves as Jews, and defined as such by the local majorities, sometimes ethnic, sometimes national, sometimes, religious amongst which they lived), regardless of their distal origins or their genetic homogeneity, have been a continuous, ubiquitous, and mostly oppressed minority on earth for many centuries.
Ironically, what really is an "invention" is not this oppressed community of fate, but their oppressors -- the "gentiles" or "goyim" -- who are defined merely as "non-Jews."
In reality, there is no intrinsic, homogeneous population of "goyim": If Jews are Earth's ubiquitous local minority, then goyim are merely their equally ubiquitous local majority, namely, humanity, with its human nature, ever ready to mistreat its minorities (as, indeed, the Jews in Israel themselves are proving ready to be -- though under rather special tragic [but not therefore exculpatory] circumstances, being themselves the recent victims of both a collective, understandable post-traumatic stress disorder as well as a probably-irremediable "demographic problem," because their own current local minority in Israel (indigenous Palestinian Arabs) is in fact part of a much larger surrounding local majority, some of them driven into exile by the "returning" Jews in much the same way -- though not for the same reason -- that whatever fragment of Jewish ancestry was indeed historically Jewish and living at the origins of the Diaspora were themselves driven into exile in Roman times.
From the first few Google fragments I gleaned, SS seems to be of an ideologically (and morally) driven (and troubled) temperament, himself scarred by the holocaust, something of a misfit even amongst others like himself, who first turned to Marxism and anti-imperialism for closure; but this ideology could not retain his conviction or give him solace; then he had (perhaps) some problems of rebellion and social alienation, partly because of his experience in the Israeli military; soothed awhile by academia (specializing in a remote and perhaps somewhat hermeneutics-prone sub-domain consisting of the history of a French Marxist), he then researched and wrote this [apparently amateur] book, which has some grains of -- familiar -- truth plus a lot of ideologically and hermeneutically driven almost-conspiracy theory to the effect that both Jews and their origins in exile are invented.
Back to the conclusion: The premise is irrelevant. It is neither the genetic homogeneity of the Jews nor their historical origins in acts of Roman deportation and dispersal that are, or ever were, at issue. It is the continuous existence of a dispersed and persecuted minority on our planet who -- partly because of our species' relentless and equally ubiquitous human tendency to persecute our minorities -- identified themselves (and were obligingly identified by their local oppressors) as one and the same continuous community of fate (the faith and race being only incidental to their state and status of stateless minority).
The result (rather than the cause) was also a larger than usual degree of inwardness, inbreeding and ghettoization that made this dispersed community rather more genetically homogenous than they might otherwise have been. (The Cavalli-Sforza genetic studies -- I am not sure about their empirical status today -- had claimed to have found Ashkenazic and Sephardic Jews, geographically separated for millennia, to be genetically closer to one another than to the respective local geographic populations amongst which they had been living during those millennia -- but this too, like SS's racial and origins questions, is completely irrelevant to the moral conclusions SS wishes to draw from his argument that Jews were invented.)
So let me close with the intended consequence of the book: Jews and their origins in Roman dispersal are an invention, therefore Israel should be a non-Zionist democracy, with identical status and rights for all, even if it makes Jews a minority again in Israel too (since it is not their true place of origin, and they are not the true historical Jews anyway, the Palestinians are).
The gist here -- that Israel should be a non-Zionist democracy, with no special rights or status for Jewish citizens, even if it makes them a minority again in Israel too -- is completely correct, morally (and is probably the best outcome everyone can hope for -- whereas the actual outcome is likely to be far, far worse, for all concerned).
But this sad moral conclusion (for this unfortunate community of fate) is (and was) the correct one entirely independent of the nonsense about Jews being an invention! They are no more nor less an invention than any other human community that sees itself and is seen and treated by others as being a community and kind. And the result -- for century upon century of those who saw themselves and were seen and treated as Jews wherever they were -- has been relentless persecution that culminated in the most barbaric abomination of all.
That is the sole rationale and justification for this blighted minority's equally ill-fated attempt to salvage its future generations from continuing persecution, by returning to what they felt was their historic home, as a majority again, at last. Unfortunately -- both for the Jews and the Palestinians -- their home was occupied, and had been for millennia, by its current residents (as well as a succession of distal invaders). So the only way to try to put an end to the historic injustices that had been done against the Jews, as the ubiquitous planetary minority, was to do a similar injustice to the indigenous inhabitants that had been the majority in "Israel" throughout the Diaspora (though no doubt there was a good deal of geographic and genetic flux there too, likewise completely irrelevant: only facts within living experience and memory are relevant).
I, for one, think that doing such an injustice to another people, even in the interests of self-preservation and the protection of future generations -- at the cost of doing unto others anything that is even faintly like what has been done unto you -- is a far, far worse dishonor to the victims and memory of the Shoah than the other (bitter, but bloodless) alternative that is now more open to Jews than ever in their historical past, namely, to give up their identity within this dispersed historical community of fate, and simply assimilate into their local national majorities.
I say this, but (being myself a member of this community of fate), I know and feel also what it means: It means that the millennia of collective clinging to their identities as Jews (regardless of its true genetic or historic basis) at the cost of relentless persecution has all been in vain. In trying to protect their own kin and kind from the depredations of the "goyim" -- to the point of ghettoization and the ejection and exiling of those of their own who "married out," because it meant mixing their own blood with that of their immediate oppressors, and the oppressors of their parents and grandparents, because it meant breeding the future oppressors of their progeny, a dreadful prospect fully comprehensible only to the Jews, this ubiquitous minority, this community of fate -- in all these past efforts to protect and preserve their kin and kind, they had actually been preserving their vulnerability, as the ubiquitous minority, to ever more of the same.
I have made no mention of religious beliefs in any of this, because -- although they, like any beliefs, played a causal role too -- I insist on treating them as the nonsense they all are and always were, whether the beliefs were Jewish, Christian, Muslim or Hindu. Voodoo is no justification for continuing to expose future generations to persecution, any more than genes or ancient history are a justification for grabbing land from indigenous people.
The Jews are a tragic community of (historical) fate. Was their fate self-imposed (not in the sense of persecuting themselves, but in persisting in remaining Jews even at the risk of continuing persecution)? I think not. I think all human communities try to preserve their identities (and their existence), especially when they are under assault. It was simply the historical (sic) fate of Jews (sic) to be the ubiquitous manifestation of this human tendency in place after place and time after time, as the earth's ubiquitous minority (for the past two millennia).
SS's selective and somewhat conspiratorial analysis of racial homogeneity and historical origins (with its vestiges of Marxist Manichaeism and hermeneutic humbug) seems to me to miss the fundamental point of the Jewish historic tragedy completely -- a tragedy that is hardly one of its victims' invention. (And one has to be a holocaust-denier -- which SS certainly is not -- to see it otherwise. SS and his strained historiographic apologetics are merely another, abstract manifestation of this tragedy: He is too ideologically conflicted to see straight, although his desideratum -- a non-Zionist democracy -- is unquestionably the just one.)