OK, I am not sure if this is a new idea or not, but I was thinking of why the Nonmen seemingly changed belief systems post-Womb Plague.
To preface this theory, we must hypothesize that collective belief in Earwa has power. In the sense that a collective belief in something, changes the Outside and so makes it true. The gods are powered by this. The more believers, the more powerful the gods.
OK, that said, it would be my theory that pre-Womb Plague, worship of Oblivion was sufficient to save their souls. It "saved" their souls, because it their souls were out of any of the god's reach, for a couple reasons: first, there were less men and so, less powerful gods, i.e. more spaces between them; second, with less powerful gods, "hiding your voice" and dodging the gods of the Outside was probably easier, since more space meant the draw of Oblivion was stronger, or easier to find. Once the Inchoroi rewrote the Tusk though, the Nonmen were damned, a priori. This meant that even a pious Nonman was damned outright with the rise of man, dispute his avoidance of the gods, because Men focused the eyes of the gods onto the Nonmen, via the Tusk.
Perhaps this helps explain this quote:
"What did you find?"
"God... broken into a million warring splinters."
A grim nod. "We worship the spaces between the Gods."
"Which is why you are damned."
Another nod, this one strangely brittle. "As False Men."
The Aspect-Emperor nodded in stoic regret. "As False Men."
Since the Nonmen had no gods, in fact, had the opposite of gods, it was relatively "easy" for damnation to have "overwritten" the oblivion the Nonman belief system was built to ensure since there is essentially no competition.
This could also work to explain this quote:
“The Nonmen…” he said evenly. “They have taught us how to hide our Voices. How to bypass the Outside, find Oblivion.”
Eyes like bladders of ink, each reflecting the tripods across their shining curve. The fluting of gill-tissues along the neck. “You worship the spaces between the Gods…”
“Yes.”
A rasp like the screams of faraway children tangled in the wind. Inchoroi laughter. “You are already damned. All of you are already damned.”
“So say you.”
A deep chested rumble. Popping mucous. “So says the Inverse Fire.”
Aurang is not lying when he says they are already damned for their Oblivion worship. That probably did work once, but it was sundered by the Inchoroi's rewriting of the Tusk. It also could be that the "worship" of the Inverse Fire also works against the Oblivion worship, so that simply by existing, the Inverse Fire (whatever it is) invalidates the native Nonman beliefs.
Not sure if that really makes any sense all typed out, but it did in my head...