That's my understanding. Decompilation is legally protected in the US, and you can do a reimplementation based on a decompilation. Sony v connectix is aiui the precedent.

Theoretically you could clean room by having different agents/models/context windows to do both decompilation and reimplementation. This is untested in court afaik and I don't think anyone wants to spend money to find out.

There was a non-clean room reimplementation of gta3 a few years ago. The gta publisher DMCAd and of course the fans who did it didn't have any money to fight in court (and probably couldn't find anyone who would take a big complicated case on such bad facts pro bono). https://www.gamingonlinux.com/2021/02/take-two-interactive-h...