Picture a gas filled balloon suspended in midair inside a constant velocity moving train boxcar. When the train slows down, will the ballon move forward or backward?
What the heck are you talking about? The original statement you objected to talked about the weights of gases relative to rock and water, not other gases.
Incidentally, I would expect that the mass of the rubber in the balloon and the string would tend to make the balloon move forward due to its inertia.
Correct on the string; incorrect on the balloon .
You can test it yourself with a helium filled balloon inside a closed vehicle.
Centrally locate a balloon in a moving car, step on the brakes ... and the balloon will move backwards. What causes it to happen is the air inside the vehicle moves forward, thereby increasing air pressure in the front and lowering air pressure in the back ... and that floating balloon will float toward the lower pressure.