If you hire someone to create something for you (video, images, audio, text, etc.), and you then claim you made it yourself without crediting them, I think almost everyone would agree that it's plagiarism and deeply unethical. It doesn't matter if you also had some creative input, since the other party was still an author of the work. If the "someone" was actually a generative AI model, I don't think that changes anything. You still relied on a third party to have creative input, and passing it off as your own work is tantamount to plagiarism. So in summary, yes, IMO AI usage must be disclosed in creative works.