For the people unfamiliar with Flat Earth, what he's getting at is that they don't believe in gravity, because they reject the idea of the spherical Earth. Since the sphere Earth model perfectly explains and predicts gravity via mass warping space-time, they are forced to say "nuh-uh" and reject it. This is because, for the downward forces we experience to occur on a flat Earth, the whole flat Earth would have to be a disc accelerating "upward", constantly going faster and faster. We can see this isn't happening, because we'd leave the sun and other planets behind.
Their alternative explanation is Buoyancy. They say "its simple- light objects float and heavy objects sink" without ever explaining why floating = up and sinking = down. There has to be something causing the buoyancy. Something pulling heavy objects down.