Proprietary projects and clean room implementation is often done without having the source code first though, and a clean room implementation assumes its actually clean. meanwhile, most of llms nowadays cant be verifiably do clean room because the source code is probably there too.

When Linus does clean room to make Linux, does he have access to the source code, or does he need to figure out things by himself and only match the interface later on?

I'm sure both of us aren't good enough to solve this billion dollar problem, but saying its not a problem to steal someone's work, disregarding their license (even MIT requires attributions!), and releasing their mangled copy as a new product, is dystopian.