Sanderson's first law is not 'Thou shalt not introduce in new world-building lore in the sequels of a serial work.' It's a principle of foreshadowing, in which you plant plot elements well in advance of them becoming plot significant so that the reader/viewer gets an 'aha' moment when you stab them with a plot twist. (Otherwise you'd know the complete ideals of all the Knights Radiant by now and not just be fed them during climactic plot moments, but I digress.)

On the subject of Limit Breaks, there's no clear cut line on what exactly is Aether and what is Dynamis. Emet, for example, uses the line 'Mine is the Aether!' when he uses a Caster Limit Break. Likewise, it's never definitively stated what Elidibus uses to power his Limit Breaks. There is, however, this unresolved bit of dialogue from your showdown with him (Hope's Confluence):

What of it?! I have my mission! I am Elidibus! And it is my duty to steer mankind and the very star upon their true course. This I swore to... to someone. We spoke, and I swore... what? What did I...?

Ah, poor old amnesic Elidibus. I really can't help but wonder if his Warrior of Light, his summoning magicks, and his 'Limit Breaks' all draw inspiration from a certain chance encounter on Elpis, countless years before. I'll laugh if he makes that promise after discovering the cause of the terrible cry from within the earth that set off the Final Days. Perhaps in a place in the depths with lots of Creation magic.

So now let's look at the A12 example that you put out. If Hermes is to be believed, the unsundered Amaurotians were too aetherically dense to perceive, manipulate, or interact with Dynamis in a meaningful way outside a few observations made about entelechies. So how did the Final Days as a Dynamis based phenomenon have any effect on the Ancients? It affected their creation magic. And what is the modern day derivative of creation magic? Primal summoning. Meaning that if Meteion can warp an Ancient's creation magicks on Etheirys from the edge of the universe, then it wouldn't be a stretch for us to unconsciously use Dynamis inside a primal.

Part of the difficulty here is that we don't have the rules, so there probably isn't enough here for you to go off of show a contradiction just yet. Can you utilize Dynamis miles beneath the veil of Zodiark's moving celestial Aether currents? It would seem that way, otherwise the Elpis flower from Sharlayan wouldn't change colour in the present era. Does Dynamis have a mass-like property similar to Aether, or is it completely intangible? What are the Aether thresholds at which you can use/not use Dynamis effectively? If the Garleans are unable to manipulate aether but their Reapers are capable of summoning Voidsent, does that mean that Void magic is based off of Dynamis? Are there any civilizations that are actually based off of utilizing Dynamis (the Amaurotian quest Debate and Discourse, for example, alludes to another civilization co-existing at the same time as the Ancients 'across the pond' that had the capabilities to build a metropolis and a calamity of their own to deal with.) As you can see, this doesn't just have implications for Endwalker, but future storytelling as well.