One thing that is often remarked upon about anti-Zionist discourse is how it takes attributes that apply to nation-states generally and ascribes a uniqueness (or at least a special virulence) to them when applied to the Jewish state in particular. With Sand's argument we see much the same thing, but applied to the concept of "nations" more broadly. The idea of a "nation" as an "imagined community", whose bonds are constituted intersubjectively rather than representative of some eternal and pure blood tie, is not exactly revolutionary in the field. And that the idea of Jewishness as a nation in the modern sense didn't begin to form until the 19th century too, wouldn't be surprising -- the concept of nations in their modern form didn't begin to emerge until the 19th century. Jewish "peoplehood" was in all probability no more or less developed than those of other peoples across history; but that doesn't make it fake (then or now).
The very, very elementary mistake some people draw from this is that, because the origin story is partially mythological, the community is a fiction. We decide what ties bind us. None of them, in all likelihood, translate into any sort of eternal, transhistorical truth -- an observation that has no bearing on their validity. It's how people screw up social constructivism more generally -- "socially constructed" is not a synonym for "fake." Most important elements of our lives are socially constructed, that doesn't make them any less real.
Nor does it particularly matter whether any individual Jewish person can individually trace their bloodlines to someone who lived in Jerusalem in 259 B.C.E. Let's say Sand is 100% right that modern Ashkenazi Jews are descended from Khazars, who were converted by Jewish missionaries from the Holy Land. To assert that this makes anybody any less Jewish or any more disconnected from a historical or Bible-era Jewish people requires us to say that only way to be "truly" part of a people is to descend via bloodline. But that's (a) not true and (b) not something Jews have ever believed. Descent can make one a member of the tribe, but it doesn't make one any more of a member than a convert. In this, one can say that Jewish identity reached a modern (or perhaps post-modern) form ahead of the curve -- our conception of peoplehood was less predicated on a literal chain of "who begats", and instead embraced the idea of created a community and nation out of shared intersubjective bonds and commonalities of creed. Not only is this not an illegitimate form of social organization, nominally it would seem to be superior to assertions that are quite literally based off ethnic essentialism.
The normative argument Sand wants to make -- undermining the "reality" of a Jewish people and its connection to Israel by way of genetics -- is something I've criticized before (and criticized when it is applied to Palestinians, who also are accosted with charges that they are johnny-come-latelys to the territories):
I hate this. I hate this in all of its forms. I hate it when folks try and tell me that Jews don't really belong in the Middle East because they actually descend from Khazars. I hate it when people argue that Palestinians aren't a "real" people because they didn't have national ambitions until relatively recently. I hate it when Israelis are accosted as inauthentic because they have the "wrong" eye color. I hate it when people seem to think this entire conflict is properly resolved via an impossible historical inquiry into who got to the Holy Land "the firstest with the mostest".And that's the problem here. There is an ongoing debate over the genealogy of Ashkenazi Jews (a debate with far more qualified participants than professors of modern French history, I might add), but to assert that it resolves any claims regarding the legitimacy of a Jewish state requires us to believe in some very retrograde notions of statedom -- that they really are and must be predicated on purity of blood and bloodline and a belief that this volk is eternally tied to this soil. The problem being that nobody believes that, and claims of that sort have long since ceased to be necessary for the maintenance of a national identity.
It all just strikes me as incredibly primitive -- based on old-school notions of ancestral ties and bloodrights and purity that have no place in modern discussions. I'm a Levi, so I assume I descend from relatively deep Israelite strands. But who knows -- maybe there are some European converts in my family tree. So what? And if some persons with African blood identify as Palestinian and are so recognized by the Palestinian community -- good for them! It is sordid business, this attempt to police each other's racial authenticity for our own transparently political ends.
The bizarre summation of Sand's normative attack is that the only legitimate identities are those actually based on historically verifiable racial purity; Jews actually comprise a variety of ethnicities who may or may not share blood ties with ancestral Jews, therefore, Jewishness is a falsehood. To state that argument is to refute it: all identities are socially constructed, and that fact does not falsify the identity to anyone but someone more enamored of old-school racist ideologies than I think Sand would prefer to admit.