I think it's still an assumption that Solus made an assumption about what happened to the Burn in the first place. The language used to describe him finding the Burn is vague enough that I can't tell if he just found the Burn as is, or saw the Burn being created. And if he did see the Burn being created, then there's still the question of if what he saw was a primal or not. There's just no way to tell.

Yes, it's right up the Ascian's alley to get Solus to think the Burn is what happens when eikons are summoned for long enough. It's also right up Elidibus' alley to lie by omission instead of outright lying (he never lies to the WoD, he just doesn't tell them about the other option they could take). The Burn could very well what happens when eikons are summoned too much. It could be something that sometimes happens when eikons are summoned, but only under special circumstances. It could also have nothing to do with eikons at all.

What the Burn does remind me of is what little is mentioned of Meracydia. There was lots of eikons being summoned and re-summoned during the Allagan invasion of it and by the end of the war, most of Meracydia was a wasteland. In fact, parts of Meracydia are still uninhabitable because of that invasion. There's not a lot of ways I could see an area still naturally being that way after 5,000 years, but if most of Meracydia was devoid of aether at one point... that's a lot easier to swallow. I kinda wonder if Solus read Allagan records of what state Meracydia was in after so many repeated summonings and war and then recognized that the Burn had all the same characteristics.