Definition of what is "trance" is rather subjective I guess. Anything spacey and upbeat with a beat seems to count. "Trance" always seemed to mean IMO the dumbed down new age gay love child of techno and acid house. The beats seem rather generic and boring in so many of these tracks to me overall, I rather define trance somewhat as a negative term even if there are many tracks there I wouldn't mind hearing.
I never really thought of Orbital, Orb, and Underworld as trance, but I can see how they influenced later trance artists. Richard D. James would be amused that he made the list.