Well, that's the assumption I'm challenging. That does not seem to be a substantiated fact at all.

Most of the arguments I've encountered online that attempt to make the case that LLMs do have a writing style that's readily distinguishable from human writing are deeply afflicted with confirmation bias, and are often based on circular reasoning.

My comment above was a rhetorical question intended to point out the fact that LLMs are specifically designed to mimic the patterns of writing found in their training data, so the idea that they'd all converge to some other set of pattern is not by itself plausible.