"Space giants" from a culture that had no concept of outer space? That sounds to me like some New Age silliness, or "re-interpretation" if I was being nice
Reading right out of the Torah in Hebrew the word is "nefilim", which means "the fallen" (from the Heavens, presumably) and is rendered as "the Titans" or "Giants" in most English translations of the Hebrew Scriptures. "Giants Fallen From the Heavens" is more-or-less the same thing as "Space Giants", isn't it?
We know that the Nefilm were halfbreeds -- the offspring of fallen angels ("the Sons of God") and human women. How exactly was this twisted mating accomplished? Nobody knows, of course, but with a bit of imagination one can speculate that perhaps it occurred through technological means, implying a society far more advanced than is usually imagined for that early period of history.
As I said: I'm no scholar. All of this is pure moonshine. Still, one must admit that there are an awful lot of funny things about the Noah story.
Everything before the flood is up for grabs. Presumably the book of Genesis as we know it was compiled rather than simply dictated. This is completely consistent with Divine inspiration as well as the mythological explanation.
If compiled, this would explain the allusions to stories that were known in more detail at one time. For example, Tubal-Cain inventing metal working, but only mentioned in passing as part of a genealogy. Or Enoch, who was "taken". There's even a whole apocryphal literature about Enoch, but this dates from much later.
The pious explanation is that the details of these stories are not necessary to understanding God's purposes. But they must have been known at one time, at least as stories.