The Burn being the result of Primal summoning isn't set in stone. Solus believed that was how the Burn was created, but it's never specified how he learned that. I'm currently 50/50 on that being what actually happened or that being an idea the Ascians mentioned to him... it certainly is a convenient reason for a lot of Garlemald's expansionist ideas. Which lead to more Primal summoning overall...

I'd find it rather odd for the idea of a primal's aether returning to the land/Hydalen to be wrong after all this time. It's introduced in 2.0 and then reiterated by Moenbryda when she talks about how Ascians are different then Primals which are different then "normal" beings. Actually, in 1.0 we see Dalamud absorbing the Primal's aether when they die and Loisouix acting like that's not what is supposed to happen to a primal's aether. So the aether should logically be going somewhere that isn't Dalamud. Storytelling convention being what it is, we've had plenty of time and opportunity for counterarguments to the life-cycle of a primal's aether to come up and no one has ever disputed it.

If anything, Stromblood has more evidence for a primal's aether being like the aether of anything else when it dies. The Lvl. 70 SMN quest reveals that what Summoners can do with Primal aetherforms (make egi based on it) can be done with the aetherfrom of anything. The thing making the egi just has to have been around for the death of whatever they're trying to make an egi out of. That's the entire reason the Allagan Node is trying to kill the WoL in the Lvl 70 SMN quest. It wants to kill you so it can get your aether and make an egi based off of it.