true, based strictly stratification....However, that's not really what I'm talking about, I'm talking CO2 moving UP and out or air moving in and pushing the CO2 out. Nearly impossible. if the area between the top of the liquid and the opening is full of CO2, there
may be a small amount of outside air that gets in, especially if it's not a narrow opening like a growler or a bottle, but the molecular weight of the CO2 prevents the majority of the air from displacing it. Sure, you're going to get a small amount of displacement, but not a total exchange to cause a major short term issue with oxygenation, and certainly not in the short period of time between filling and capping/sealing. But, hey, to each their own level of care with such things. We all do things a bit differently.