Quote ‘Yet another possibility is that Romania has an undersampled ethnic group that overperforms, but whose schools aren’t tested very well. The only group this might be is Romanian Jews’

Let’s say that there are probably many ways of interpreting this sentence, but I can’t find any nice one. I understand this as ‘it makes sense for a group with skin color X and hair color Y to be better at school’ followed by ‘it must be Jews’.

What am I missing ?

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.

You're not missing anything. The author is a "race scientist".