I am more or less a post-modernist, though I arrived at my understanding of the core ideas independently of the authors cited in this article (though on reading them find myself nodding in agreement). So I appreciate this clear and articulate description of postmodernism, and the argument offered against it. And I am sympathetic with the observation that the postmodernists did not exactly make their position clear. To me, it is clear, but you have to step through a rhetorical mess to get to it. It's a bit hard to do in one paragraph, but let me try:
Let's take the criticism offered by Erazim Kohak to the effect that "tennis balls do not fit into wine bottles". How is this not a fact? he asks. On observation, it is easy to see that tennis balls do fit into wine bottles, but the context here is of trying to squeeze it into the bottle through the opening. Now what has happened here is that the problem has been framed in such a way as to allow only one way for a tennis ball to 'fit' into a wine bottle. But why would we frame it that way? Why do we privilege Kohak's description of tennis balls and wine bottles and how one fits into the other? Once you ask that question, you become a postmodernist.