He appears to think that they are related somehow to consciousness itself, possibly material beings at times and immaterial at others, possibly from a parallel dimension.
In at least one other book, he proposed the hypothesis that they (the occupants) are in some way related to various figures (mythological and otherwise) in our past (like elves, fairies, monsters, demons, etc.).
The consistent theme I find through Vallee's books is that these beings (whatever they are) are liars and tricksters.
God does miracles, angels use technology ... and there are good angels and bad angels. Both use technology. If lies and deception are detected, it’s the bad angels. For an example of encounter with good angels, I offer the Travis Walton ‘abduction’ case. He was accidentally slammed by the propulsion system of a craft, then the occupants took him on borad to aid his recovery. He was ‘gone’ for several days. His experience is the fictionalized Fire In The Sky movie ... very fictionalized. There are several youtube vids of him speaking of his experience.