I do not know how the authors of "The Dark Gods" got to their conclusion but...
Both UFO visitations and demonic visitations can be explained by the same phenomena:
Paper: Sleep Paralysis, Sexual Abuse, and Space Alien
Abduction
Paper: Hypnagogic and Hypnopompic Hallucinations during Sleep Paralysis: Neurological and Cultural Construction of the Night-Mare
The Nightmare, by Henry Fuseli (1781) is thought to be one of the classic
depictions of sleep paralysis perceived as a demonic visitation.