They actually have surprisingly good comebacks for our proof
Proof: The Earth casts a round shadow on the Moon during lunar eclipses.
Comeback: That shape could also be made by a disk.
Proof: All the other planets and stars are round.
Comeback: Earth is special.
Proof: Because of the way gravity works, the closer you get to the edge, the steeper the gravitational pull toward the center would be, meaning you and all the water would fall to the center.
Comeback: Gravity doesn't exist. We just move up at a very fast rate that pushes us to the ground
Proof: The way light bends around the circumference proves Earth is round.
Comeback: ...Oh no youre right