[kybernetikos]

This seems like you don't believe that AI can produce correct new work, but it absolutely can.

I've no idea whether in this case it directly copied someone else's work, but I don't think that it writing good unit tests is evidence that it did - that's it doing what it was built to do. And you searching and failing to find a source is weak evidence that it did not.

[bsder]

There is no way on this planet that an LLM "created" the exact unit tests needed to catch all the edge cases--it would even take a human quite a bit of thought to catch them all.

If you change the programming language, the unit tests disappear and the "generated" code loses the nice abstractions. It's clearly regurgitating the Python code and "generating" the code for other languages.