I think what you're proposing isn't something they can do. Are you saying "What if I asked it to create a short story who's pieces don't resemble any pieces of known stories?" or are you saying "What if I asked it to create a short story who's whole doesn't resemble any known stories?"

The first one can't happen. The second? Yes, it's stealing.

Where is it getting this story? LLMs don't have creativity. They don't understand story structure. It pulls sentences and paragraphs from work in it's training data. If the generated output contains work that others have made, that's called plagiarism. If it doesn't, then your hypothetical isn't realistic. LLMs can't create original works. That's the whole point. It pulls pieces of the training data and rearranges them. It would be like if I was writing a college paper and instead of writing anything myself I just pulled 100 different sources and copied a sentence or two from each source and structured them as my paper. That's 100% plagiarism.