Hum, Ayleids ruins are known to be in Cyrodiil, so if you see a screenshot with an Ayleid ruin in the background, you can be sure it's in Cyrodiil. As said screenshot show no sign of jungle anywhere, then we can conclude that Cyrodiil will be the same as in Oblivion. His assumption seems legit.
That said, I hope that this ruin was an exception and that good parts of Cyrodiil will be covered in jungle.
Ayleids were also extensively in Valenwood. Not saying the screenshot is from there, but still.
It would not be a retcon of a retcon. In Skyrim, Heimskr (the preacher in Whiterun) quotes Tiber Septim from when he turned the jungle into forest. So Bethesda sort of officially admitted that Cyrodiil was not always a temperate forest.
Only it was. Talos made it so that it had always been a temperate forest, retroactively. And a jungle. Gods are funny that way.
EDIT: The video crapped out on me after the ten minute mark. Any interesting lore stuff he mentioned?