The author also writes a blog of the same title. Here's a somewhat recent entry from the blog, responding to John Bolton describing President Obama as "Post-American.
There is a difference, of course, between arguing that Obama’s policies are somehow post-American-exceptionalism, and saying that Obama himself is post-American. The insinuation blatantly is that he is not really American, a notion that will doubtless give succour to “birthers”, racists, and other nidiots. Indeed, one might even suspect that the phrase is designed to do just that, to act as a little slow-fused bomb of hatred and contempt secreted within an ostensibly rational discussion of foreign policy in a broadsheet newspaper. I don’t recall any Republican wingnuts arguing that, say, Bill Clinton or Al Gore were not actually American; presumably there is something about Obama other than the colour of his skin that renders the accusation more relevant in his case, though I cannot right now think what it might be.