Yes this isn't clean-room. Though none of these decompilation projects have been resolved in court yet. re3 (GTA3/Vice City decompilation) developers were sued by Take Two but they settled out of court.

https://www.pcgamer.com/take-two-dismisses-lawsuit-against-g...

I didn't know about this. Not sure if the developers settled or take two gave up. I would guess the latter as the decompilation / port scene seems to be going strong. Though I don't follow it that closely.