I'm fairly confident that it's not just plagiarizing because I asked the LLM to implement a novel interface with unusual semantics. I then prompted for many specific fine-grain changes to implement features the way I wanted. It seems entirely implausible to me that there could exist prior art that happened to be structured exactly the way I requested.

Note that I came into this project believing that LLMs were plagiarism engines -- I was looking for that! I ended up concluding that this view was not consistent with the output I was actually seeing.