...wait, why do you suddenly decide this one part is the result of a lie, when we take every single other thing said both by Ifrit and the priest in this scene as truth? Why is this one part suddenly the thing we don't believe?
That scene is actually the one singular evidence we have that already being tempered is, itself, a defense against tempering. We have never once, in the entire game, seen an attempt. This fact is actually the lynchpin of the entire 'WoL is tempered' theory, too; since the theory presupposes that the real reason were immune to tempering isn't the Echo but the alleged tempering*, we have to consider the one source of this fact to be legitimate.
...so why do we believe him about that one thing, but not every single other thing they say, including the thing that shuts down the theory? Because where I'm standing it just looks like you're picking and choosing the facts you want to be valuable and claiming the evidence against your theory to be nonsense.
*Incidentally, on an unrelated chain, this theory also dismisses Zenos, Arenvald, Fordola, Mikoto and Misija, all of which have the Echo but no Blessing of Light, but who all resisted tempering on at least one occasion (fun fact, Mikoto's the only one that's only done it once). Misija outright says that it's because the Echo is the thing that protects against tempering, with her evidence being Ysayle--but since Ysayle actually did have the Blessing, she's not on the same list. So, yeah, we've actually got a significant amount more instances of 'Echo, no blessing' people resisting tempering than we do people with the Blessing doing the same.