It's not an ad hominem. "You're not an anthropologist, so what you said is wrong" would have been an ad hominem. The GP actually attacks the argument, although

It's not an ad hominem. "You're not an anthropologist, so what you said is wrong" would have been an ad hominem. The GP actually attacks the argument, although somewhat incompletely (to be polite). EDIT: The authority is actually named, it's "anthropology".