Being completely untainted is the standard many reimplementations set for themselves to completely rule out legal trouble. For example ReactOS won't let you contribute if you have ever seen Windows code. Because if you have never seen it, there can be no allegation that you copied it.

That is however stricter than what's actually legally necessary. It's just that the actual legal standard would require a court ruling to determine if you passed it, and everyone wants to avoid that. As a consequence there also aren't a lot of court cases to draw similarities to