Even 90% of the rain would have had the same effect. Florida's problem is more about the vast numbers of people moving in and living close to the water. All these new communities shed their water, instead of letting it seep into the ground. They might have some retention ponds, but those fill up and flow out when you get any significant amount of rain. They are typically sized to handle a few days worth of the everyday thunderstorm, not the sky tsunami that happened.
Where does it flow to? To those older communities that did not plan on having to deal with all this excess water. Hence the "We weren't in a flood zone" mantra you hear from most of the people who were flooded out. Were it not for the new communities, the old communities would be in much better shape then they are now.