I guess you have to drain the swamp first then it will work. ;)
...
Or you could try and change
<terrainAffordanceNeeded>Heavy</terrainAffordanceNeeded>
to
<terrainAffordanceNeeded>Light</terrainAffordanceNeeded>

Open the mod folder via the mod menu ingame and look for the "Terrain_TS.xml" to edit it to your liking.

Edit: I did just that and it works for me, saves me a lot of time in my mountainous and at the same time swampy region without even having the tech for the pump. xD

Thanks, I'll give it a shot.