I'm not sure how true that is, actually. The 1e monster books absolutely integrate psionics where appropriate.
I mean, they put them in there, but we actively ignored it.
It appears that's what GG preferred? (Dragon Magazine #103 from November 1985 about his envisioned 2nd edition):
Random resource on Psionics in D&D:
Canonfire! - Users Journal
www.canonfire.com