Don't know if it matters, but the carapace seems to be an aporetic object itself and not just a case with chorae stuck in it
Seswatha glimpsed the unholy Carapace shining in the whirlwind's heart, a nimil sarcophagus sheathed in choric script, hanging...
Warrior Prophet, US edition, p 143, just a little into chapter 7.
So maybe it's whatever a big chorae would be? A bigger contradiction/paradox? It's a pretty sweet set-up: can't be attacked by convention forces, whirlwind; can't be attacked by sorcery, aporos; can only get to it with a laser rifle, and it has to be big.
Any significance to the nimil? Maybe we'll get a nimil revelation in Ishterebinth.