I have a feeling we’ve asked Nephilim if they had mana before, and they did not. I can’t remember the details of this, but the “having no mana” part was something I was fairly certain of when I wrote these notes. The theory part is that they weren’t affected by the anti-mana because they have no mana of their own.
The concepts of spirit, soul, and mana are very tightly tied together in many of our notes. Its a very strong theme in the Elsicara Glyphs too. Others say the soul is the container for mana, but the association is still quite strong.