They don't believe in gravity in the same way that we do. They believe that the Earth is accelerating upwards at 9.8 meters per second squared, thus emulating gravity.Gravity is not exactly 9.8m/s across the entire Earth ...
If it was a disc accelerating it would be constant across Earth's surface.
BTW after a mere decade accelerating at 9.8m/s2, disc-Earth would be close to the speed of light.
Colliding with interstellar gas at that speed would irradiate Earth : we'd be cooked.
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