They don't consider the object being heavier than the surrounding medium as a legitimate excuse for it falling down because they begin with the presupposition that the earth is a globe and that up and down do not objectively exist and are relative. So without the magic of gravity there is no reason why the heavier object would fall "down" because as i say, up and down is relative to them based on their globe assumption.

We have an objective up or down in our presupposition, so we dont need any other forces to come in and explain away anything.