My first thought was using it as an aerial equivalent of sea serpents, but A. most D&D settings have few to no airships and B. sea serpents don't need to be human-plus-intelligent.
If they did, the Charisma score would need to be sane (and not just a +16 bonus to AC).- No Str, +10 Dex, +18 Con, +4 Int, +8 Wis, +32 Cha. It's pretty hilarious how absurdly high you have to pump that charisma in order to make an incorporeal creature with a decent AC. To give you an idea, even a Great Gold Wyrm only has +22 Cha. Malcanthet, the freaking queen of the succubi, only has +30, and Aphrodite herself, the most charismatic god of all, only has +36. Even better since literally none of the worm's abilities key off its insane charisma.
I feel confident arguing that the tunnel terror is all mouth.
On a primal level, I like this backstory better. It's funny.
But when I take it seriously, I prefer the version where Kyuss gets screwed over from his own arrogant callousness and not random chance. I get wanting to spite Nerull, though.
Maybe if Nerull told Kyuss how to become a god, forked over some important resource, and Kyuss screwed it up on his own? Would that be a good compromise?