The eye opening thing to me is not the inflow or the storage, but rather the fact that they held the primary spillway at 60K CFS while the emergency spillway was being topped... they had so little confidence in the integrity of primary spillway that they were willing to let water flow over an emergency spillway that had never been used before.
My theory is that they intentionally tested the emergency spillway while they still had an operative main spillway to end the overflow if problems were found..