Well, I think it depends on how much effort the 'writer' is going to invest. If the writer simply tells the LLM to write something, you can be fairly certain it can be identified. However, I am not sure if the 'writer' provides extensive style instructions (e.g., earlier works by the same author).

Anecdotal: A few weeks ago, I came across a story on HN where many commenters immediately recognized that an LLM had written the article, and the author had actually released his prompts and iterations. So it was not a one-shot prompt but more like 10 iterations, and still, many people saw that an LLM wrote it.