Garet - I didn't think to check for posts until after I had posted the quote. I'm apparently quite self-absorbed.
It seems that the 3 Gnostic schools that would have learned about this from the Nonman Tutelage were all wiped out in the First Apocalypse. The Mangaecca moved to the Incû-Holoinas, and the Mihtrul and the Sohonc were destroyed.
Tirtiga, who IMO would be the only one who achieved hiding his voice in this manner, "died" almost 1,000 years before Seswatha was born.
Seswatha founded the Mandate (the last Gnostic school of the time) after the First Apocalypse and could have possibly not known about the practice of worshiping the spaces between the gods as a method of avoiding damnation. Either that or, worshiping the spaces between the gods only grants oblivion, and Seswatha wasn't after oblivion.
But it would seem to be quite crucial information, and not something the Sohonc likely would just forget or don't care about. And that would imply that Ses knew, too.
I see a couple of possibilities:
1) The Sohonc didn't know at the time of Seswatha. Why this would be, I can't say. Maybe they were almost obliterated at some point, and someone never returned that particular book to the Sauglish library.
2) The Sohonc (or the Mandate) knew, but at some point their elite damnation research team has proven the practice ineffective (project de-funded and merged with the Various Odd Nonman Ideas Catalog project). So Ses and the Mandate know but don't really care a lot.
3) The Sohonc knew, but Ses forgot to tell the Mandate, or more interestingly,
deliberately chose not to tell the Mandate.