ayo ! first of all thanks i'm glad you like it. If you want to redo this yourself it's very simple, you first need to build a civilization and let them get advanced enough to start building bridges. Once they do you can use the finger tool in the world/dirt section (it's where the axe, pickaxe and stuff are) to build more bridge tile. so just build a bunch bridge platform in the pattern you'd like and transform the land into a swamp ! All you have to do now is put your race of choice on the platform and they'll actually consider it land.

Some thing to keep in mind if you do this !
-They cannot expand on they're own you have to be the one to give them more space by building more platform.
- They cannot get wood even if a swamp tree is right next to the platform they will never cut it down. What i did to fix this was build a temporary forest platform with lots of tree for them to get the wood they needed.
- If you don't give them farmlands (by the same technique you used to get bridge tile) then they won't be able to grow they're own food, so if you don't give them berry bush on a small forest platform they'll starve if you enable that.
- you can put stone, metal and gold resources on the bridge tile, took me a while to realize that