My understanding is gravity is generated from the molten core of a spherical planet.
So a FE could not have a central "gravity" point. So a resonable explanation is, the FE is moving
at a speed that confoms to known effects on mass.
This then is different from all observations of our neighboring planets. I would argue that humans have invented some powerful telescopes.
The findings in just our immediate 8 planets and thier 173 moons is that they are all spherical and have a gravity like force.
Let me ask what precludes FE's to believe the earth is different?
Newtonian Gravitation arises from any mass, but the Gravitational constant is extremely small, so only very massive bodies produce a significant gravitational field. The shape of the body does not matter, though a spherical body is simple to handle as outside the body (eg earth) all the mass can be assumed at the centre.
The disc-shaped flat earth has to deny Newtonian Gravitation simply because the gravitational field of a flat disc is nothing like that of a Globe earth. One of the biggest differences is that gravity at the edge would point towards the centre (or hub), not "down" normal to the surface as we actually experience.
So, Rowbotham hypothesised that Newtonian Gravitation is a fake, and what we call gravity is actually the earth being accelerated "vertically" at 9.83 m/s2.
There is a lot more to be said, but undoubtedly that will come out in the debate to follow.