Ok, so, someone used an LLM do create changes. This new code is no longer under the project license it is, as you say, public domain.
Move forward 400 commits. At what point is most of the code public domain?
While correct you still missed the point completely.
So,not make this as clear as possible: You can not license LLM generated code. Not under GPL, z lib or other copyleft licenses. It may work with public domain licenses.
For MIT licensed projects there is not a big issue.
For the kernel, have a look at the rules. AI models may assist only.