Even if the students wore American flag garb only on Cinco de Mayo, I take it that the message was “you want to stress your Mexican heritage, and we want to stress our American heritage” or at most “we don’t entirely approve of your stressing your ethnic heritage, since we should all think of ourselves as Americans.” This might convey some disagreement, but it hardly strikes me as discourteous; and to the extent that it’s a “rebuke,” it’s the sort of message that people are entitled — not just as a matter of law, but also of good manners — to send. Courtesy doesn’t require absence of disagreement. It requires that the disagreement not be framed in a rude way, and I don’t think there’s anything rude in the messages that I infer the clothes were trying to send.
I think Volokh is missing the most obvious potential message here, though, and it's one that is clearly discourteous: That unlike the Cinco de Mayo celebrators, the wearers here are real Americans. Volokh's claim that the message is that we should all view ourselves as Americans is, under this view, precisely the opposite message. The students may have been intending to send (or the recipients may have received, regardless of intent) the message that "people with your cultural heritage and background are something alien and external to 'American', which is what we represent."
Or maybe not. But it's a very plausible reading of the t-shirts, and one Volokh doesn't address. And needless to say, that message would be quite morally pernicious.