Saying Solus "conquered" the Burn is a bit inaccurate if nothing lives there... but I digress.
The problem with his assessment is that it's an assumption, not a proven truth. If there's anything to be learned from my bumbling here, it's that operating on assumptions and faulty logic can lead to cascading inaccuracies. While it is readily apparent the Burn is an aether-starved wasteland, nobody has an eyewitness account of primals being responsible for it, and for all the warnings we've been given about them the worst we've seen any primal do to the environment is Bismarck eating a few islands in the Sea of Clouds.
While it is plausible for primals to have been responsible for the Burn, how probable it is remains unclear; it could be the result of something else or otherwise a natural phenomenon of some sort. While primals are the most likely culprit, there remain other possibilities that shouldn't be discarded just yet. Personally, I hope those other possibilities are closer to the truth than the obvious, and not just because I'll be beside myself if the Imperials' attempt at global conquest is running on an incorrect assumption. (It's mostly because I like mysteries and being left guessing, though not quite to Kingdom Hearts' level of everything coming out of left field. Given this game's track record, though...)
Also, I'm only here because I got knocked down Heaven-on-High like a wrecking ball.