My gut says you're correct. Ideal conditions need time for phenomenon to occur. The O2 is going to get swept out in the rush as part of a mixture of gasses. But co2 settling (if it actually happens) would also take time. Probably hours or days for a significant co2 layer to form on top of the beer.Stratification of CO2 is a myth at the scales we're dealing with. You need a perfectly calm column of gas on the scale of kilometers for gravitational effects to have a significant effect on a gas blend.