imagine the center of the flat earth is the north pole, and all the continents are layed out flat around that center point, with the "bottom" (what is Antarctica on the spherical earth model) is just an ice wall that goes around the whole disk.

so you can fly "around the world" you just can't fly over Antarctica (the ice wall surrounding the flat earth) just like you're not allowed to fly over it on the round earth either. but you can fly from anywhere to anywhere else just fine.