It is plainly stated by God himself in Genesis that there are two seeds... serpentkind and mankind.I'm aware of the Nephilim @TokiEl
What strikes me as "reaching" is the "hollow earth / hybrid monsters waiting to break out any moment" idea.
It may be the case - I believe God parted the Red Sea and everything else, but as far as interpretations based on narrative derived from the Bible, I am more cautious. You don't get to have been a Christian for as long as I have without having been wrong about a few things.
I tend to divide my Christian ideas between foundational truths (the cross, the resurection, the Trinity etc) and biblical opinions (where the Garden if Eden was, what the seven thunders might have said etc etc)...
As for the Nephilim, there is a thread I started up a while ago to share ideas so I welcome your perspectives...
https://vigilantcitizenforums.com/threads/הַנְּפִלִ֞ים-the-nephilim.3721/
And this information has been suppressed by governments around the world... where ufos and alien abductions are well known realities. We can speculate why this is suppressed but if you work for the Serpent why would you not work for his seed as well ?