It's the same model as the idTech GPL releases - the code is open source, the rest of the game is not. To legally play the original game or a modified version of it then you'll still need to buy it for the assets, but there's nothing stopping you from only taking the code and building a brand new game on it like various studios have done with idTech (e.g. Selaco and Wrath: Aeon of Ruin).
Yea, this seems -fine- to me. Even if they had to rip out some third-party licensed code that they couldn't open source[1], to the point where the game wouldn't even compile! Some code is always better than no code. The open source community can/will fill in any gaps.
1: Which seems to be the case here. To fully compile, you need:
DirectX 5 SDK, DirectX Media 5.1 SDK, Greenleaf Communications Library (GCL), and Human Machine Interface (HMI) “Sound Operating System” (SOS), or disable the code that calls into them.
