The stance is not to 'prevent AI-assisted code to reach their codebase.' It's not like AI-assisted code is literally poisonous and their codebase dies if touched.
The stance is to deter random vibe-coders trying to resume-max by submitting PRs to known open source projects. There are so many of them rn. Hopefully by making it clear (some of) them will realize doing that is just wasting their tokens.