a bunch of people get together, rewrite something while making a pinky promise not to look at the original source code
guaranteeing the premise is basically impossible, it sounds like some legal jester dance done to entertain the already absurd existing copyright laws
Clean room implementations are a jester dance around the judiciary. The whole point is to avoid legal ambiguity.
You are not required to do this by law, you are doing this voluntarily to make potential legal arguments easier.
The alternative is going over the whole codebase in question and arguing basically line by line whether things are derivative or not in front of a judge (which is a lot of work for everyone involved, subjective, and uncertain!).
I've always taken "clean room" to be the kind of manufacturing clean room (sealed/etc). You're given a device and told "make our version". You're allowed to look, poke, etc but you don't get the detailed plans/schematics/etc.
In software, you get the app or API and you can choose how to re-implement.
In open source, yes, it seems like a silly thing and hard to prove.