> If I as a human want to make a clean room reimplementation of some API or application, I must not have read the source code of the original implementation.

That is the difference between necessary and sufficient. Clean-room is sufficient to guarantee avoiding copyright, but it is not necessary. The line legally is south of there, but that position was chosen because they didn’t want to crossing and it was easier to argue for legally in court.

tl;dr: clean room is overkill for avoiding copyright infringement