I believe the climate in Cyrodiil changed gradually in the last thousand years. In the time of Alessia Cyrodiil was a jungle, but over time the jungle gave way to the forests as seen in Oblivion.
You don't have to make things complicated for them to work.
Peleus, Talos just retconned it all instantly. So in reality, one day it was jungle, next day it was forest.
I believe there is a discrepancy here. I believe Peleus is taking that approach because there is a certain level of skepticism around CHIM and other lore, especially concerning the Septims. People could say it was their Emperor that did it for them, but we can't say for certain. I far prefer the skeptical approach to lore rather than the believing everything the books say. Authors are often wrong you know, there is a very good chance your science book right now has quite a few things that will be laughed at a hundred years from now.
But if you do want to use the retcon, it had already happened before Morrowind, so it would be a jungle if you are doing that.
Wasn't that statement a dev question dodge anyway?