If you gave me natural language from a 125 IQ human and a 155 IQ human I'm not sure I could judge which human is smarter from the writing quality.

And that's a massive intelligence gap!

It may just be that writing is mostly saturated, and the remaining perceived gap is not quality but some kind of "cultural fit".

IQ is a useless thing, but you can absolutely tell intelligence from writing. You don't need to write like [author of your choice], you need smart ideas and the desire to make them understood.

AI writing sucks because there's no intent or choice behind it. It's human thought, blended by linear algebra into pink slime and extruded into sentences.