Just because it's open source it doesn't mean there's not a person or group of people deciding what happens to the 'main'/'upstream' version of it.
Sure you can fork it, and remove all AI contributions, but who will use your fork?
When people say "the Linux kernel", they mean the version maintained by Linus Torvalds. They might mean a fork for a specific SoC or manufacturer or distro, but those are all just the Linux Kernel maintained by Linus Torvalds plus a few patch sets. There's no considerable divergence.
Linus' stance was initially "no AI written code". And he does have final say in what goes into the repo he maintains. Now he's changed that tune, he allows AI written code, and no matter how many forks you make to strip it out... what matters is Linus' upstream.