that's considered official lore, it was a jungle
No, it doesn't count as official lore since it was in a book. It counted as possible lore. In-Game Books cannot count as official cause that would be like saying that books about Dinosaurs in real life is official when the truth is, we have no idea...
Just because a book in Tamriel says that doesn't truly mean it is official.. Like I said, the writer of that book could've been someone who never saw a forest/jungle before and called it what he wanted, which is highly possible.
Actually, it is like the "Argonian Account" books, if I'm correct. The books about an Imperial going into Black Marsh and he talks about how bad it is down and how he doesn't want to go down there... Yet if an Argonian wrote a book about Black Marsh, they would most likely say how wonderful it is for their people and go on about the hist, and such... Same thing here, a guy wrote a book about Cyrodiil, or w/e, and said it was a jungle.. And if someone else wrote it, they prolly would've said something different.
~Edit~
I think this the major problem with people... They read a book that is in the game and think it is official, instead of thinking of the possibility that it is incorrect... Seriously, Bethesda had set-up their books in Tamriel just like what they would be in real life... And I love that.. More life-like.