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

[Brinne]

How do we know they’re sundered ancients and not say, some of the sundered new life that sprouted after Zodiark restored life to the planet?

That's possible too! That's why I said "at least generally" since, again, the Sahagin are shown able to carry the ability explained as a sign of being a former Ancient soul.

I will certainly also take Emet-Selch being so conflicted over feeding the Namazu to Zodiark he misconstrued that as being the source of the Hydaelyn-Zodiark war. Either way is hilarious.