"Not suitable" is a designation RCID came up with to characterize land they can't build on... without a lot of hoopla, or, may never be able to build on if they don't get permission from the proper state agency. It is arguably poor nomenclature. It should have been labeled "Conservation Set-Aside."
It is land that is designated for conservation. And they legally can't build on it without permission as per their original agreement with the State of Florida. And to get that permission, they need to buy a heck of a lot more property off-site and put that aside for wetlands conservation. Which is what they just did to put up the new DHS parking lot in a conservation designated area which had been labeled "not suitable."
The land WDW originally identified to be the area set aside for wetlands conservation tends to be low lying swampland in the 100 year flood plain. It's land that's difficult to build on, but not impossible. Once they do decide to build on it, they need to: drain it; put up or rearrange canals and re-/detention ponds; and then, put tons and tons of soil on it to raise it up out of the flood plain -- just like they're doing for the DHS parking lot.
Not all of WDW is swamp, there is plenty of land well above the water table, as well as plenty right at or under the water table.
See this thread for the maps:
https://forums.wdwmagic.com/threads/maps-of-the-reedy-creek-2010-2020-plan.935133/