I think the Apple doc is slightly inaccurate: the OpenStep part which include a graphic user interface was not available on HP-UX but the non graphic part: WebObjects, Enterprise Objects Framework, and Portable Distributed Objects was available.
Looks like Wikipedia has avoid history of OpenStep:
http://en.wikipedia.org/wiki/OpenStep
Searching around, I find quite a lot of pages that mention NeXT's implementation of the OpenStep API for HP-UX, but none that mention that it's only the non-GUI parts. Are you sure?