I have seen what you are talking about. IIRC, it is just the Windows 95/98 version of DF (identical to the DOS version except it as a bare bones frontend that only works if you install the game in the default directory anyway). The JK 3 Level Demo I got with my Monster 3D II card a long time ago. It's just the first three levels of JK's single player, identical in every way, including cutscenes, to the full game. No multiplayer. At least the online demo had one SP and one MP level (the cutscene was a crappy montage.. the SP level was Level 9 Fuel Station Launch and the MP was Bespin Mining Station). It also had limited skins.
So my advice.. just buy the cheap DOS Dark Forces or whatever version you can find. And download the JK demo or buy the full game for cheap.