ChatGPT cannot form a novel argument. It is a large language model, trained to regurgitate language in the patterns it was trained on. It strings words together based on probability, calculated based on what has been said before.

But for argument’s sake, let’s say ChatGPT stumbled upon a unique idea that was both interesting and accurate. The author would have to cite ChatGPT as the source of that idea, otherwise yes, it is plagiarized.