And it doesn't foreclose the possibility of LLM achieving human-like writing ability. If I stole the writing of a human author much better than me it would still be plagiarism.

Of course, with real books we do have the problem of ghostwriting, where an author willingly writes for a book that will be published under someone else's authorship. That may be where things end up here, with requirements to acknowledge sources, whether ghostwritten for you or written by others.