I don't see a problem at all with that statement.
The internal system of the free body diagram could be an elephant, stone or what ever you like.
The droping of anything is not an equilibrium system its accelerating(dynamic). After x amount of time the system energy isn't equal to each other, where as with our boundary conditions the light aircraft will have the same system energy as the heavy aircraft after a period of time. I know the Kinetic energy will be different but the Ek will cancel out because the speed of the aircraft will be the same at the start and end of the time period which is one of the the boundary conditions "constant speed"
If you can't see that an accelerating object is a dynamic system and our discussion is a static system in equilibrium your missing the basic foundations of my argument.
You can sex up what ever you want but it doesn't change the fact its techno waffle of "if you can't dazzle them with brillance baffle them with bollocks". And take it from an expert of "baffle them with bollocks" thats what you are doing.