The GPL license no longer covers the kernel in legal terms.

The uncopyrightability of AI-written code only applies to the actual strings of code generated by an AI, not to the entire project.

A person could ignore the GPL if they only copied the AI-written portions. But, how could they know for sure which lines were AI generated and which were not? A wrong choice would leave them civilly liable for copyright violation and all they stand to gain would be tiny portions of the Linux kernel code which are worthless by themselves.

There's no reason to steal the AI generated portions and risk a lawsuit, when you can just generate your own code.

There seems to be legal discussions about that. It’s not quite as simple as you say:

However, there may be cases in which a different assessment is justified, namely when users use and operate the LLM as a tool that merely implements their personal creative intent. This could be compared somewhat more vividly to using a paintbrush. If the brush merely rolls over the paper, for example because it is dropped, no copyright-protected work is created, even if paint remains on the paper. However, if a painter deliberately swings the brush in a certain way, a protected painting can be created. If AI is used in a comparable way a copyright-protected work can indeed be created.

https://kpmg-law.de/en/ai-and-copyright-what-is-permitted-when-using-llms/