True, bu 1) GB as a country has lots of immigration, 2) there are those Asian wizards sporadically mentioned before. Maybe the martial arts master whom Voldemort killed was another of Salazar's grand(...)children, and the girl living 'where they don't get invitations to Hogwarts' is yet another one?

(Or maybe she's a resurrected, Obliviated and Time-turned Hermione. After all, Quirrell didn't say he will restore her to Harry, andsuch a resolution would, from Harry's point of view, equal the destruction of Hermione's self and so a sacrifice large enough... Though that would violate restrictions on Time-turners).

On the other hand, it seems redundant of them to have discussed the possibility of a dying wizard making an Unspeakable Vow, and an opportunity for it not arising before the end).