I think you must not have interacted with him much, or have not talked with people who know him more closely.

I was explicitly advised to avoid applying for my PhD at Northeastern unless I had incredibly thick skin to withstand the way he berates his students. So I didn't apply — I'm not signing up for that.

I had pretty uniformly positive experiences with him as both a student and a TA for one of his classes, but the very important bit of context there is I was a student who did well enough in his class to be asked to TA. In retrospect I can remember him being pretty awful to some of the other students.

Matthias is virtually always the prickliest person in the room at academic programming languages conferences, by quite a bit.

I remember seeing a talk about typed racket (by samth? Probably...) and he expresses a very personal view, something like "I think types are an important tool to enforce correctness of code" and someone in the audience said "I think you're full of shit!". Everybody sighed a short sigh and everything continued as usual.