Someone going to public library, copying every book out of it and then selling them as their own are thieves.
No, they've merely infringed copyright. Theft is a well understood concept and it does not pertain to information.
Okay, but that's not what happened as far as I'm aware. Who did they sell their copies of the books to?
Not technically theft unless the public library no longer has the books. Selling something and saying you created it when you didn't is a different five-letter word: fraud.
Putting prompts in an LLM and saying you "created" the image or text thereof is fraud.