I'm not so sure I buy that. AI written text is fairly obvious to good writers with exposure to LLM output. Is it a case where it's sort of an average of writing styles, but that average is not human and thus humans can detect it?

AI writing you can recognize as AI writing is obvious. Newer models are better about this and the line will only get more blurry. Here's a benchmark where good writers make the assessment rather than different LLMs ranking each other: https://surgehq.ai/leaderboards/hemingway-bench

The top models are also the latest:

Gemini 3.1 Pro: still a bit of a gremlin, but will probably stay on top until the other model makers go xkcd 810 and target this benchmark

Gemini 3 Flash: current favorite of writers using it as a helper for its speed and decent prompt following