From my understanding, it wasn't so much the rainfall/flow as it was the damage to the spillway.
If the spillway doesn't experience structural failure, there's no problem. But it did, so flow through the spillway had to be reduced, which led to the emergency spillway coming into play and they had to continue using the damaged spillway, failure or not.
If the spillway doesn't experience structural failure, there's no problem. But it did, so flow through the spillway had to be reduced, which led to the emergency spillway coming into play and they had to continue using the damaged spillway, failure or not.