I guess its not the models which have to be more high-resolution but there's some kind of displacement map for the textures. For complex models like the dragon this displacement map is created from a high-res model, of course. But for textures like the stones or the walls I think they tile it, otherwise it would be quite inefficient to create content.

What I don't get, though, is why this should be DirectX 11 only - aren't geometry shaders already present in DirectX 10?