And who said Van Gogh is a genius?

basically everybody

Why?

because... history?

he was a Genius, but his IQ was probably average, according to what we know about him

Hard to say. He was nearly as good a writer as a painter and presumably wasn't even trying at that craft. He would be remembered for his letters even if he never painted a thing, and they were purely accidental because Theo wasn't always in town. But you still have a point.

I don't know the history, that's what I was asking. I googled before I asked but everything I could find seemed relatively split with many sources implicitly or explicitly referring to him as a genius.