The point of a clean-room implementation is that the only thing passing from A to B is a detailed specification written from scratch, because functional details cannot be copyrighted.

Which I think you know, so I don't understand your argument.

You said in another comment "However, this specific thread is about the scenario where an employee has inside knowledge and is passing that knowledge on to the implementer."

It being proprietary and/or secret information doesn't affect the copyright of the clean-room output. If there's trade secret implications those are worth discussing, but as a separate issue.