It's not an uncommon meme that programmers and mathematicians possess a weird twist in the mind that allows them [in the strong form of the meme] to do, and [in the weak form of the meme] to enjoy doing, what they do. If you grant that, then it's a small step to admitting the existence of different sub-twists that incline you towards different parts of the abstract space. If you don't grant that, then consider it a rhetorical device for pointing out that comparatively few people like Haskell versus JS.

Why the word "defect" though? There must be other words that are less controversial. Maybe "abnormalities" or "specialities"?

How about "possess an outlier brain design" among the human or even general IT population.

Because less controversial words would be less funny.

"Interests"? "Appreciations"? "Proclivities"? "Personalities"?

I read it as more of an assertion that loving any language requires cognitive defects, some a bit more than others.