It's not depending on the CSPC engine, it's using it's data.
Ok, it reads where to put what NPC and stuff, but theoretical you can edit/change the data files and create your game. Without the need of CSPC at all.
Open-Source engine and a clone... you can call it the same.
But it's a new engine, behaving like the original CSPC but like I mentioned above, that can be changed...
Then, would you call The Underside and Hack9 also clones of CSPC?
especially Hack9 is "just" like CSPC, and don't tell me it's a mod...
It uses MP3s and other stuff we would never dream of...