Acutally it uses both. DirectX is a combination of a set of input, sound and grapghic API. UHLE uses DirectInput for its input, DirectSound for its sound, but doesn't use D3D (Direct3D) for its video, it uses glide for its video. So what I was saying is that it would be difficult to port it to linux beacuse you'd have to rewrite quite a bit for UHLEs source, the parts for input (which is very small) and sound (which is quite large). Also Linux uses Glide API a lot differently then the Windows Glide API.
Not to mention that you'd have to completely re-write the GUI in QT or GLADE (or some other GUI program for Linux).