The Voidwalker tanks all of the phases except for the fire elemental phase, because that requires far more finesse than what you can do with a pet, or they can do on their own. The Voidwalker also didn't tank the demon phase, because there wasn't one. Their gear levels combined with the new talents of patch 3.0.2 and the added raid nerf let them beat the encounter without ever seeing an entire phase of it. That's like opening a bag of Runts to find out someone took all of the bananas. The bananas are the best part, you jerks.