I wonder if they had enough material from individual humans if they could've distinguished between them as well? It really seems like their model is learning to recognize some general form of writer's "voice", so to speak (and I assume their final layer just knows which voices are supposed to be tagged as what).

I heard an author say recently (I think it was a blog posted here) that an LLM was able to identify him from one of his unpublished high school essays.

The DoD claimed to have de-anonymized Satoshi Nakamoto by similar means a while back. (Well, I think it was before LLMs. By similar means I mean stylometry, running statistics on a person's use of language.)