That is just a way of simplifying the situation to demonstrate that the math would be the same in either case.
But imagine being inside a sealed container floating in space. As long as you were moving at a constant rate of speed in the same straight line direction, ie, in a state of inertial motion, you wouldn't be able to tell if you were in fact moving at all. It's only when an outside force is introduced (energy is applied) that you would be able to detect a change in your situation. Even then, you wouldn't necessarily know if the container was being drawn to some planet gravitationally or if there were some force pushing the container in a particular direction causing you to accelerate.
Similarly, if some outside force caused you to change direction, you would detect that as well, via the centrifugal force. ie, your weight would shift to one side.
But energy is always being applied. Every single body in space produces a gravitational wave, imparting some kind of pull on you and the container. If the container is a different mass than you, you will be able to measure the difference in the force exerted on the container and the force exerted on yourself. Floating in the center of the container, movement would occur as you saw the container react to the forces applied to it and to you.