Agreed, the NAB has terrible commentary that makes one wonder if it’s even written by Catholic editors - at least ones on the more liberal side of theology.
I think the point that the Nephilim represent a baser part of humanity is very valid in the Jewish understanding of narration.
It’s also not impossible that these are derived from neighboring pagan tribes - the idea of a demi-god or half-celestial/half-terrestrial race, not unlike the Greek titans. In this narration, though, they aren’t seen as beings to be worshipped but rather a fallen part of creation from which God must cleave His people, and hence the Flood. The Covenant with Noah and with Moses was one where holiness is separation, where animals to be sacrificed and people’s lives were ones marked by separateness and dedication to the Lord. The Nephilim would provide a good contrast for this analogy of the Covenant.
Lastly, I heard Bible Answer Man try to tackle this one - he walked right over it and called it essentially irrelevant and gobbledygook.