In fairness to the topic, not all "alien astronaut" theories buy into Zacharia Sitchin's patent medicine. Erich von Daniken is probably a more widely-received promoter, who focuses more on evidence than on causative theory.

But the Nephilim, et alia, were all wiped out by the Flood, and everything we know of the history of civilization occurred after the Flood, so there is no real room for "demons" in our history.