I don't think he used the invincibility globe. Wouldn't the BFG destroy?
Even without it, though, killing all those enemies with the BFG seems feasible if they started close enough to you. If you were in a small room--and you targeted an arch-vile each time--between your two shots and the splash damage, you would most likely kill both arch-viles, as well as all the revenants. Any that you didn't kill would be close to dead. Then you could just whip out another weapon and clean up the rest. I'm not saying it's something you could rely on, but with a little luck you could definitely pull it off.