Leaving aside the question of whether that combination of images is novel… In your example, all the proposed novelty is specified in the prompt. ChatGPT didn’t come up with it, you did.
> Isn't it pretty much what any person would do, mash up some related words?
Yes, that’s exactly what chatgpt does, and it’s what many humans do. But to be analogous to your initial example, there should be another person who actually came up with the instruction specifying which words to mash into a poem. The word-masher, whether human or chatgpt, is just following the instructions, not coming up with them.