We cannot know, in the same way we cannot know that it doesn't contain code that is hand-written on graph paper and scanned in via OCR.

The standards for code submissions for the kernel are extremely high, and their review process very strict and complete. There are no barriers stopping LLM generated code from entering the code base, but the barrier of entry for the code quality itself is so high that you have to submit code at the quality of a seasoned and competent engineer.

Ultimately, does it matter that the code was LLM written if the quality is sufficiently high?