The classical definition of a clean room implementation is something that's made by looking at the output of a prior implementation but not at the source.

I agree that having a reference compiler available is a huge caveat though. Even if we completely put training data leakage aside, they're developing against a programmatic checker for a spec that's already had millions of man hours put into it. This is an optimal scenario for agentic coding, but the vast majority of problems that people will want to tackle with agentic coding are not going to look like that.

If you read the entire GCC source code and then create a compatible compiler, it's not clean room. Which Opus basically did since, I'm assuming, its training set contained the entire source of GCC. So even if they were actively referencing GCC I think that counts.