They definitely interpret things a bit differently. The main achievement here is that all the code required to run the game is now open, the game no longer jump

They definitely interpret things a bit differently. The main achievement here is that all the code required to run the game is now open, the game no longer jumps into original game code from the original Dungeon Keeper executable in order to function. Whether the source in itself is 100% legal is not part of that equation.