Throughout the entire Old Testament, "bene elohim" means only one thing: angels.
The Sons of Seth hypothesis is nothing more than a weak modern literary contortion developed to deal with the uncomfortable story of fallen angels mating with human women.
BTW, the ancient Hebrew stories are mirrored in other cultures of the same era. Ancient Sumerian and Mesopotamian stories line up with the Hebrew Book of Genesis (albeit with different protagonists) as do stories from ancient China and India.
All this said, what you want to believe is your business.
Therefore there is evident for this claim; right?
Well; if we believe that the Bible is true; these other kingdoms arose AFTER the Tower of Babel story.
Since the language was confused and people separated; the memories of what had occurred earlier was surely shared by all; thus there would be a thread of commonality between ancient stories.