True, which is why they don't apply Newtonian rules to the flat earth. In their world, gravity isn't a thing. They go to great pains to differentiate between "gravitation" (acceleration for no apparent reason) and "gravity" (acceleration due to the attraction of mass to mass). It takes a while to get your head around it.
Actually, they (FEers) use the concept of "Universal Acceleration" to explain what the rest of the world interprets to be gravity. Gravitation is used in FE mythology to explain an entirely different, yet equally paradoxical and mysterious, phenomenon. Gravitation is defined thusly by Dictionary.com:
gravitationnoun
1. Physics.
the force of attraction between any two masses.
Compare law of gravitation.
an act or process caused by this force.
2. a sinking or falling.
3. a movement or tendency toward something or someone:
the gravitation of people toward the suburbs.
Hence, if they choose to "create" another, gravity like, force, they will have to choose another word as "gravitation" is already taken. It (gravitation) makes no distinction regarding the types of mass which experience its force, hence the earth, which has mass, would experience it to the same extent as would any other object of the same mass. If they choose to evoke a force (other than classically understood gravitation) which acts between all celestial bodies and the earth, they must call it by another name, given that gravitation is reciprocal.
That very reciprocity introduces even more complications and complexities into their model than those with which they presently have to contend. So please, FEers, find another word with which to describe your newfound "force". Even a "made up" word would be more acceptable than gravitation. It would certainly be more appropriate.