Yeah, that’s why all the IBM clones had to write their BIOS firmware in clean room implementations of new software that implemented the same functionality as IBM’s own documentation described.

Functionality can’t be copyrighted, but code can be. So the easiest way to prove that you made something without the copyrighted code is to mimic the functionality through your own implementation, not by transforming the existing copyrighted code, through decompilation or anything like that.