Doesn't he just say that Romanian Jews overperform?

I don't think there's a need to read the authors words in the worst possible way.

> I understand this as ‘it makes sense for a group with skin color X and hair color Y to be better at school’

It wouldn't make sense for the author to mean it that way, because the "white skin black hair" classification likely includes more non-Jewish than Jewish Romanians.

> followed by ‘it must be Jews’

I mean, it's not that hard to believe. If you look at (Eastern) European history, A LOT of the people of scientific or cultural significance were Jews. Don't pin me on the numbers, but if you check the Wikipedia article of a random Easter European person that left a mark in science, there is an 33-50% chance that person has some sort of Jewish background.