Wouldn't using OpenAI break the cleanroom?
The legal framework around all this is way too immature to make a definitive statement one way or the other. Anyone who says otherwise is speculating.
The only way to clarity will be legislation or court cases, because it really could go either way.
You can't claim that some work is necessarily tainted by existing, you also have to prove which knowledge was retrieved, used and that there was no other reasonable way to know it. But unless it is actually tried in a court in all major jurisdictions, it's just speculation from all parties (including line).
One person could also claim that someone someday might leak information to them, and even though they'll ignore it, since they'll be exposed, they are tainted and can't contribute anymore. It'd be absurd.
A project is free to chose who can contribute, but they can't necessarily gatekeep others from doing their own project with different rules.
It doesn't have to necessarily have to make sense to make a claim, but it doesn't necessarily bear any weight.