It’s definitely one of the theories I’ve seen. Checking on The Flat Earth Society website I see it’s not their official position (but they acknowledge some groups do believe that). They don’t have a different theory though, just that “objects simply fall”.
The sun and moon are tiny and relatively close. They rotate in circles above the surface at a constant distance (don’t ask why). From what I know they spend a lot more time “disproving” existing theories than finding new ones to explain their model.