Well I hate to break it to you, but the book was written by a fiction writer. There's no such thing as the Imperial Geographical Society, it's all made up and make believe.
Well, I hate to break it to you, but I do realize that the book is fiction and was in fact written by Michael Kirkbride and not the Imperial Geographic Society. It's called speaking from an in-game perspective.
And if Bethesda wants to change the terrain of their imaginary world in order to better suit a game setting, what difference does it make? Just make up any justification you want to explain the change, it doesn't matter.
There seems to have been a misunderstanding of sorts. The only things I have claimed is that Cyrodiil being a jungle is not just a Morrowind rumor, it has been mentioned in lore books as well. That's it. I have until so far not expressed any sort of judgement of their decision to change the settings.
So I suppose this would be a good time to actually go ahead and express my judgement. ^_^ To me Elder Scrolls games are primarily about exploration and for that they require an interesting setting. And I think that a jungle Cyrodiil would have made for a more interesting setting and therefore a better Elder Scrolls game. The vast majority (maybe over 90%) of all fantasy settings are placed in a temperate pseudo-European medieval world so it's always refreshing to experience one that isn't. And that is what difference it would make.
But I understand that different people have different preferences and what I would call boring someone else might call familiar and what I might call exotic someone else would call weird.