> Well that is already how it is done with numerous multi-decade open rewrites of closed games

Serious such rewrites don't start with the code of the closed game!

> I don't know how this squares with law, but Oracle v Google gave a very valuable judgment to the public that an API is not copywritable. If we take the LLM out of it, that's all we are talking about in the pure case.

Not at all. The LLM used to write grit has seen the git code. That is what we're talking about here.

> Of course, we can't take the LLM out, but it is the starting point.

The LLM isn't the important thing. The important thing is that the git source code was used to make grit.