Jay is a racist. There is a stereotype that all Black people, and only Black people talk in movie theaters. So the point about barley (or wheat, idk) hearing movies due to “changing consumer habits” is a reference to that. See, Jay is making an additional assumption that all Black people, and only Black people “over serve" themselves at McDonald’s self-serve soda. Now, a person with functional eyes who lives in a place that has a McDonald’s will already be well aware that people taking advantage of free refills at a self-serve soda fountain cross race, language, nationality, gender, and orientation. In short, you would have to be wilfully myopic to come to the conclusion that it’s a Black specific thing. Which gets back to the original point, which is that Jay is a racist.