"[D]oes Yglesias really believe that the GOP won't try to capitalize on Obama's past history with drugs?" he asks. Of course I don't (though I doubt the GOP will do it directly, it's more an "independent expenditure" kind of thing to do), and I don't think that's what I said. What I said was that I don't see anything racist about inevitable attacks on Barack Obama's drug use.
I think there's every reason to believe that a white candidate whose memoir strongly implied a past history of cocaine use would face attacks for it.