If they get a lot of rain at once and have some serious flooding, how high they are above sea level won't matter.
I'm not saying they'll be closed forever (the poster who said weeks or months is overdoing it, IMO), but considering how easy spots of WDW can flood from just a thunderstorm passing over, concern over flood damage in the parks isn't crazy.
Also, whether WDW handles the flood waters well is one thing. Potential flooding/damage where cast members live could limit employee attendance or their ability to even get to work.