Huh? Yeah, he does use the model. Look at all his animations on the WoWhead modelviewer. They match up with all the screenshots and video previews we see at Blizzcon. Garrosh is to the new Orc model as Varian is to the old human model. It's a custom remodel of the new skeleton. Also saying that the new Vol'jin model is what the new troll models will look like, is just like saying that the new orc models would look like cataclysm Thrall or Garrosh before they were released. Vol'jin is a remodel of the old Troll skeleton. I'm confident it'll look trash compared to the new model when it comes to its motion and limbs.
Edit: http://www.wowhead.com/npc=71865 . Click view in 3D and check out all the animations. They match up. Garrosh is the new orc model.