My guess is the flood icon is a glitch... Features being placed on other Features is such a big gameplay upending that I imagine they would have mentioned it. I also can’t see anywhere in the video where a builder can plant forest on floodplains, and it’s not obvious to me anywhere in the video that floodplains contain forest or jungle. Do you have a time stamp?
0:35, you can see the icons
edit:ninja'd by 8housesofelixir
Worth noting tho: its not a matter of features on other features, it's a matter of floodplains no longer being the only feature to flood. so now we have to make the distinction between floodplains tiles and tiles that flood; they're no longer perfectly equivalent groups.
I've been playing with a mod called "cheat map editor" for a while, tho i rarely use it, esp for anything major. Though I have noticed that using it, you can put floodplains tiles where-ever, and make tiles that used to be flood plains no longer floodplains, but none of this changes where floods occur. The part of the game that determines where there's floodplains can operate separately from the part that determines where floods happen, it's just that only now is that functionality in the base game.
The forum is 
