as I understand it, the predominant flat earth theory states that there's an ice wall surrounding the edge which is guarded 24/7/365 by NASA-employed gunmen
which, if you think about it, is way more ridiculous than just saying that the disk has some sort of strange periodic boundary condition whereby you teleport to the other side when you pass through the barrier on the edge.
if we assume the world is flat, it's not much of a cognitive leap to just say the edge is a magical portal either