What's the point with that quite lackluster story in Endwalker?

[Absimiliard]

I've never actually heard of the term myself, so I'm one of the people in that camp, lol. On preliminary research, it seems a fairly neutral term typically relating to human extinction in some form. It may not be the best fit here, though, since it appears to describe scenarios that occur passively or indirectly as a result of human action rather than actively requiring a perpetrator. You could say what Venat did resulted in in the omnicide of Etheriys, but not that she committed it, I think?

I believe it's one of those terms that can work with or without a perpetrator. The term really seems to be a catch-all for extinction level events with a discernible and presumably non-natural cause. I grabbed this little tidbit fram Merriam-Webster's:
"As our understanding grows, so does the horror. What we face now is not merely slaughter or genocide, but "omnicide," the obliteration of all humans and all living species.—Penney Kome and Patrick Crean"

For our purposes here, I'm of the opinion Venat would indeed qualify as a perpetrator of omnicide. I'm certain plenty of others disagree, but that's kinda my take on it. While it can be argued the shredded up constituent aetherial and physical matter of the world's inhabitants ultimately coalesced into new beings, the simple reality is that those are not the same people or creatures they were. Those people are gone forever, as are all of the old world's other inhabitants. It's certainly extinction of a sort. My own personal philosophy leads me to feel this is in fact worse than simply being killed in the conventional sense, but that's a different discussion entirely.