Based on, but as far I remember it was rewritten rather then copied.
To be honest, the should claim same with PoP and I don't see any solicitor suing them, as it would be too much hustle to prove its a code copy instead of inspired by.
Wow that is really sad, we live in the world where this is even a discussion. Where a love project that will not be commercialized has to think about this bs :(
Same with OpenRCT2, which I've heard is also based on the disassembled original, yet people seem to turn a blind eye.