a true gravity bong would be filled with water, a hole releases the water, drawing smoke into the container, if you get the drain flow correct yop get very dense clouds.

Glad I'm not the only one who thinks it should've been called a vacuum bong

My guess is when we packed a bowl and lifted the bottle you could see it raise the water inside the bottle, then watched the water drop down, via gravity but not really, as it sucked in the smoke. More to due with pressure differences as you said.