1. We want to guarantee our work is not a "derivative work" of anything Apple wrote.
2. If we look at any Apple binaries, there's no way prove that our code didn't borrow from Apple.
3. Since we didn't look at any Apple binaries, then there's no way our work can be a derivative work-- we didn't even look at their stuff.
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.
It doesn't have to necessarily have to make sense to make a claim, but it doesn't necessarily bear any weight.
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.