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Flatearthers just want Earth to be the center of the universe and for humans to be special. They worked backwards from that goal to the Earth being flat despite all the evidence otherwise.
If you could convince them there's other flat planets, and it's inhabited by non-humanoid aliens, their interest/belief in the Earth being flat deteriorates. Since you can't convince them that, just keep asking them to humor the idea and speculate about the different flat-planets and alien life no differently than if they were other real planets and watch as everything they say just turns into quoting the Bible.
The idea of sapient alien terrifies them enough that it must be disregarded as "demons" or some other nonsense like a government spy-op.