I want to preface this post by saying I think it has zero practical relevance to homebrewing. It's just a thought experiment, or maybe knowledge for the sake of knowledge.
I have heard many different people say that oxygen will not enter the headspace of a flask of starter wort that is actively fermenting because it's under positive pressure from the CO2 outgassing.
However, we know from Kunze (page 611-612 in the 5th edition of TB&M) that oxygen can diffuse through a crown capped bottle of carbonated beer: "Even with the crown cork, however, oxygen uptake through the compound can generally be expected, just as surrounding smells can be imparted with time into the beer flavour during storage of bottled beer - despite the difference in pressure between bottle interior and surrounding air."
If oxygen can enter a capped bottle of beer under positive CO2 pressure, wouldn't it also flow into the headspace of a flask of starter wort that is covered with a piece of aluminum foil, even if the starter is actively outgassing CO2?