While tank DPS is not required it helps farm groups in a variety of ways. Faster kills equals more loot per hour. Higher DPS equals less mechanics. Fights like t12 get extremely quick and easy with very high DPS. 3 Bennu make it hard to lose, same goes with three fountains.
Behemoths tanks typically parse under 200 which is well below the minimum acceptable performance, so I can understand his PF comment.