Then the PR can be evaluated, rejected if it’s nonfree or just poor quality
I don't get the difficulty of rejecting "if it's nonfree or just poor quality or known LLM code". I don't think it's a vague criterion.
And for many projects, if you admit it's from a StackOverflow post, unless you can show it's not a direct copy they will reject it as well. This isn't commonly taken as incentivizing people to lie.
Now whether you think LLMs are worth the trouble to use is a different discussion, but the enforcement point doesn't convince me.
There is also a responsibility and liability question here. If something turns out to be a copyright issue and the contributor skirted a known rule, the moral judgement may look different than if you knew and included it anyway. (I can't comment on the legal outcomes since I'm not a lawyer.)