Actually it's not quite as simple as that, it all depends on the source and distribution of the water, compared to the location of the drain.
If the water table all around the car park can get all the way up to the deck level, then I agree, the drain does nothing.
But say the leak was in the middle, right under the "drain". Then, there is not much pressure left at the leak site, just the head associated with its depth. So it might take some time to saturate all the ground under the deck, all the way to the edges. One reason the water table rose so quickly and effectively might be that it was driven by 3 bar or more pressure from the main.