[dudeinhawaii]

It usually refers to situations without access to the source code.

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.