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

[PawPaw]

Where was it said that any Ascians were tempered? I remember that being a popular fan theory, but I don't recall any place in the game that says it.

Endwalker, in fact, outright contradicts it by saying that "tempering" was a side-effect diliberately invented by the Ascians when they taught summoners how to mimic creation magicks. That's why Livingway was able to fuel the Ragnarok with non-tempering summons, which she describes as summonings as they were originally used and intended.