It is possible to play a buzz roll and still play around the rest of the drum kit.... because as the name implies you are making the "buzz" by forcing the stick into the skin and not allowing it to naturally rebound, so if you practise it enough you can get to the point were one hand will play the buzz, the other will play another drum, then swap, then back again.....
Great drummers like Virgil Donati and Thomas Lang can do this so fluently that it's sickening, and they aren't doing buzz rolls but actual double strokes rolls which are harder because of the higher element of control needed to keep the sticks rebounding only once.
Then again, it is possible that he overdubbed the toms (Mike Portnoy's been known to do this for effect) but it's also possible that he actually played them