As it has already been stated (and also backed up from a bethesda member), the game takes place in skyrim, so, there obviusly are mountains.
Now, please, people, answer me this; if you HAD to put mountains in a sandbox game, which obviusly work as some kind of surpassable "barrier", how would you place them? Would you simply place them randomly, or would you see an opportunity in it and place them so they gently guide the players towards the direction you feel they'll have the most fun (aka: main quest), even if its not mandatory for them to go in that way?
When I heard the "We wanted to add mountains to block the player and control the experience" quote, what sprung to mind was: Yes! Tighter gameplay!
It's a shame the TES series is no longer first-person only, because if they didn't have to worry about third-person animations, they could quite easily bring climbing back and allow players to scale mountains themselves instead of forcing them up set paths.
I kind of had to answer to this, haha. As much as I'd love to see some climbing, I believe that saying that the fact that the TES series are also avaliable in third person takes away the possibility for climbing, is a bit of a stretch.