My worry is that what we recognize is the unstylized default, but it might be very easy to emulate any particular human easily enough with some examples and a prompt and n-gram/style template biasing token selection, or iterative generation-scoring loops.
Yeah. The big LLMs have fairly idiosyncratic writing styles. Smaller finetuned LLMs can be quite different (and you can also tune them on a particular person's writing). I think some of the confidence in this is because there's plenty of cases which just don't get noticed.