Your anonymous statement is not law.

Then why can't you or anyone else establish that empirically (i.e. scientifically)? Purely coincidence?

But wouldn't that require the particles to be at rest, for their weight to affect objects (other particles) below?

Not really, no - but you can conceptualize it that way and everything works. Gas behaves as a fluid. Do fluids ever stop moving (brownian motion, thermal variance, atomic vibration, valence orbits etc. etc.)? Do fluids rest and have weight?

In a rigid glass jar, there is no active pressure being pushed on the gas from the jar because the jar is rigid. It's only the pressure that the gas exerts on the container.

It's newtonian relativism. You may soundly conceptualize either, and or both forces as newtons third law requires you to.