The earth is not flat but the trees are very helpful for the survival of the planets ....
I guess by "Flat" certain FlatEathers mean a round Plane(Planet?) in a bubble rather than an Earth that has surface shaped like orange or ball.
* Better analogy from flatEathers: it's like a round and transparent/glassy Fishtank (glass represents Earth's Bubble) while the water and solid part inside the fish tank represents Earth's surface
* Compared to the one from modern Astronomy: Earth's surface (both ocean & land) is shaped like a ball.
One sounds reasonable, the other sounds ridiculous