Oh, okay, so there's no interest whatsoever in finding textual foundation. Is it fanfiction if you're clearly in no way a fan of what the writers wrote? ...and also not actually writing anything. What's the 'minimum viable product' for fanfiction as a term, anyway?
Since there's no substance or evidence to any of this, I choose to ignore most of it, and instead try to 're-reail'.
It is sort of a curious absence, especially now that you recall that we had 'afterlife-epilogues' from all three of those characters in Endwalker and its patches (even if Lahabrea's was somewhat nonlinear). It's a surprising gap that we never got either a story about that moment, or someone talking about it. The writers usually love circling back to Emet-Selch!
Perhaps it's sort of a functional element: by nature there must be a loophole for survival, but it doesn't really matter what that was. It's not the only one in the game, in retrospect; we don't know how the eventual founders of Gelmorra weathered the Great Flood, after all, even if we know the effective escape plans for every other successor civilization. Perhaps it also hits a problem of implicit magnitude: a means of escaping the Sundering feels too potentially huge to be a passing element, like it has to be important if it's established at all. That's either a technique with enormous potential (on either side), or a location left unharmed by a planetwide shattering, and either one sounds like it should be important. And since it's not actually important, therefore, it must never be definitively established.
To a degree, it's almost the most developer-sanctioned answer to this thread subject: a handwave specifically designed so that you may ignore it. And it's not perfect, but it might be better the way it is: it would sort of destroy the integrity that Emet's entire story is based on if he hid while all his people faced the nebulous-but-conceptually-horrifying events of the sundering, so he must therefore at least partially not be at fault. Meanwhile, Venat's story already has the splash of darkness of doing the Sundering in the first place and blaming herself for the ensuing suffering (as we saw in The Answers Walk), doing something horrible for noble reasons; a bit of 'and she might've deliberately spared people who'd go on to be real ratbags' isn't a big change to her overall colors.