KAM,
Get your money back.
I kid because I love, of course. Your earlier formulation was much stronger -- between your college roommate's wisdom and "I don't see what's wrong with saying...." "most Jews" turned into "some Jews." OF COURSE you can convert to Judaism, for example, and you cannot convert to "Arabism," it has (as you point out) always possible to marry into and out of most tribal groups.
The larger "myth of race" and "myth of ethnicity" would also founder... the fuzzy edges of a group cannot be treated as explaining the bulk thereof. But that's another argument. Work now.
Dan