Good question. Can you ask an LLM to repeat the entire contents of a novel, word-for-word, and read that instead of the original book? I haven't tried it, but I would guess it would not be able to do this.

Can you ask it questions about the book and expect it to get them right? Yeah, probably. Same as if I read the book and you asked me questions about it. The LLM would probably answer those questions better than I could, and about every single book in its training data, but still same-same.

I don't think this is plaguarism.