> ...the odd grammatical error convinced me that it wasn't the result of someone asking an LLM...

That's easily solved by models intentionally introducing the odd grammatical error here and there, just enough to convince the sceptics, not so many as to give the impression of being unlettered. A bit like the mythical 'RHS button' (which stands for 'real human shitty' but in reality is called the 'Shuffle' or 'Swing' function) which is supposed to make mechanically-precise drum machines sound more like human drummers.