Two quick points for consideration.
The term "nephalim" is Hebrew, and it means 'those who fell', meaning those who fell from heaven.
Also, the head binding to elongate human skulls in children, is not a cause but rather it is an effect. Head binding is intended to emulate something that these people saw and revered. If these beings with elongated heads were super-intelligent and leaders of communities, it follows that primitive people would have concluded that their intelligence comes from the shape of their skull. And, if they shape their children's skull in that way, their children will be intelligent and revered as well. Skull binding in primitive peoples is their way of imitating and sort of carrying on, something that made a huge impression on them.