The wording for it is also clearer in French in that it is referring to spreading Zodiark's energy - as we know from 5.4, primal tempering is alignment to the primal's aether, which is consistent with this (and in the case of the beast tribe summonings at least, a rendering of the soul in a more umbral state): “That’s how the Ascians became the energy transmission belts of Zordiark, my young friend! Our existence has only one purpose: to amplify and extend the part of Darkness in the whole universe.”
As curious a phrase as it is, I've never taken that quote to imply he couldn't change course - he did after all in 5.3, when he comes to the WoL's assistance in SoS (really, as far back as 5.0, although this was more debatable back then but became clearer with post-5.0 interviews.) Fandaniel too was able to go as far as assuming control of the primal - and lest anyone say he's sundered and thus the Aetherial Sea would wash this away (ditto with Emet in 5.3 after his 'death'), Emet-Selch had another quote to the effect of the Ascians preferring the souls of the original summoners because faith in Zodiark is seared onto them, so that it's far from clear that this would remove tempering. Moreover, Yoshi P makes zero reference to tempering in discussing his motives, e.g. here, or in any other interview of which I am aware. Ishikawa attributes his downfall to his kindness. It's a red herring to me. They have had plenty of opportunities now to refer to tempering as the reason and did not, much as they have had ample opportunity to pitch the third sacrifice as significant to Venat's motives, and yet they have not, instead citing dynamis manipulation and the Plenty.
While I will note that the other language versions of the Loporrit dialogue are clearer that Zodiark could temper, it's attributed to his sheer power. At the end of the day, he's not summoned with the flawed Primal rituals the Ascians created, so I think that Emet's line doesn't really mean much in the grand scheme of things. The primal was there to deliver their salvation and not lord over them, so in my estimation, they may have retconned it, or it may have never meant that much in the first place - it's hard to say. By 5.4 at least they'd already put in place the notion that tempering and a sense of memory and identity can work against each other and the sort of tempering he effected just doesn't seem to involve subverting their own motives through an act of will.
My assumption on why they brought in tempering at all is to explain cases like Nabriales and Lahabrea, the latter of whom is more zealous (and also suffered significant erosion of his identity due to body-hopping), as opposed to Emet-Selch, who is clearly fighting on behalf of his people, and Elidibus, who is doing the same but in the context of tolerating memory loss in order that he can complete his duty as a Convocation member without sentiment interfering. I am sure they wish they could've re-written Lahabrea with the original plot in mind. I just don't think they worded it well enough when he brought it up, but I am glad they never invoked it to explain Emet's motives, because they were strong enough as they were without the need for such a thing.
Of course this is all based on present lore understanding, and they could always introduce more "surprises" in the future, for better or for worse.Their writing could benefit from being clearer as there were still a lot of ambiguities for what was meant to be the final chapter of the original arc, and yeah, some of it does come across as handwave-y retcons.