Yeah, but in the context of "America" meaning USA (1776+), "Mexico" must also mean the nation. If he meant indigenous peoples, then you have to include those in what is now the US, too.
Bottom line is that had this been the previous President, the Press would be making MY argument, be sure
Obama's gaffes are just misstatements—"we know what he meant, but he's very busy, and has to talk to a lot of people, etc." When Bush did the same "he's not that bright."
I think in general most such misstatements (by all Presidents, yes, even Bush) are actually just accidents that happen when you live under a microscope.