"The demons were then the spirits of the giants when they were destroyed by the Flood."
 

I would have to respectfully disagree with both you and that so called study. Whoever posted that had not really made a study of demons, yes there are fallen angles yes God ditroyed the world in part (mostly I beleave) because of them mating with women and have children and corrupting the human blood line because they were not fully human. However Demons existed long before that point.