All that processing is done by the tablet device and only the basic game info is sent to/from the tablet. There is no remote rendering being done by Xbox console. Hence your confusion with power requirements.
Wait a moment. Do you want to imply that a smartglass device runs a "cut down" version of the game(The Division in this context) which covers top down drone perspective and they only sync the positions of dynamic elements? That's hard to believe because a 2nd render target sounds far less complex.