We sort of explain it in the opening section of the planets section, but we should probably address it there as well.
As far I can tell the FE model does say that the Planets are bound to the sun somehow (maybe with gravity). Explaining it with systems that all assumed the earth is round.
The modern explanation of the solar system is based on the "assumption" there is gravity and with this the earth is orbiting around the sun like every other planet in the system. With this (and probably more information) they calculated the sizes of the moon, sun and the other planets. When there is gravity in a FE model then the sun would be about as big as the moon and the other planets equivalent smaller about the same rate. And the distances would be much smaller. Also the earth would have to have more mass in the center than on the edges which would be a measurable difference in gravity.
The conclusion that Earth is a sphere because we see spheres is only one interpretation.
One could also interpret that the Earth is a plane because we know from human experience that bodies generally need a platform to exist over or upon. Billiard balls need a Pool Table. Basketballs need a Basketball Court. The game Water Polo generally takes place with a flat foundation with layers of an aqueous medium, with a flat surface and floating balls. It does not follow that because the balls in those games are round, the those courts are also round.
Those examples are easy to understand but have a problem with the scale of this situation.
According to RE the earth has a radius of 6.371 km and a mass of 5,972 × 10^24 kg. Lets say there is a Human with the mass of 100 kg.
Now lets take a ball with the radius of 1m it would have a mass(with the same density) about 2,3 * 10^4 kg
Compared to the Human with 100 kg he would have a mass of 1 * 10^(2 - 20) kg = 1*10^(-15) g and with the density of an human there would be a square 0.00001 cm edge length that is about to compare for the scale. If you would put an object of that size to the ball it would probably stick to it because of gravity but only as long you don't have any other gravity that is stronger than that of that ball.
Got stuck here with gravity.
What I wanted to show is that with that scale comparison the ball would be almost flat for the "human square" because it can't see the curve.