Though the chap in question appears old enough that he should have been taught the difference between adjectives and adverbs. His high IQ should mean that he has no problem learning such things, at which point being able to apply the rules in the correct way is a logical problem solving process. Clearly there’s been a failure in one of the 3 steps – it seems inconceivable he hasn’t been taught the appropriate rules when he goes to a school interested enough to carry out IQ tests, therefore either he’s not good at absorbing information or not good at applying the rules in a logical way. My understanding is that linguistics is part of IQ.
Or…. he’s been taught the rules, applies them when being formally assessed (schoolwork/exams etc.) but the rest of the time just talks how kids talk and as long as people understand him cares no more about it?
In the same way I have a different formal writing style to forum posting style, and I adjust my verbal grammar and vocabulary to the audience I’m interacting with.
Or even wackier theory…has been taught the rules, understands them but just doesn’t care as it’s not important to him.