Agreed, but still I have a hard time calling it a "myth". There are so many videos that prove gas will sit in a container at room temperature for extended periods of time. I'm not a scientist, so I need to stop embarrassing myself by trying to explain why the gas sinks, be it temperature-related or otherwise, I just know that the more of a gas you push into an already-air-filled vessel, the more of the gas there will be, thus minimizing exposure to the air once you start pumping liquid into said gas-filled container. It's like some people are so convinced that gasses mix and dissipate so quickly, they wouldn't even admit that they were crying in a room filled with tear gas. They'd be too busy shouting at me, "*COUGH* The gas is *COUGH* MIXING!! Not *HACK* DISPLACING the oxy-*COUGH*WHEEZE*" LOL It's like, if gas mixes to impotency so fast, why you chokin' homey?
Whatever makes people sleep better at night. The point is that the gas is there. It doesn't just vanish because magic. If you pump CO2 into a carboy for several seconds, there's going to be a lot of CO2 in it, and it will remain there until some other force or atmospheric pressure acts on it. If nothing acts on it directly, it will most likely take upwards of a half of an hour to fully mix with the room's air. I used to think it would take longer, but MadKing's post about air and gas mixing over time proved me wrong. That said, 20 minutes, hell, five minutes is still plenty of time to rack a beer.
So, "myth"? No. "Misconception about why it works"? Yeah.