It's a reasonable question. First, I think that the linked example is not the best of post-modern thought.

More importantly, a lot of post-modern thought is co-opted and the label is forcibly excised. Here are some examples of what I think are good post-modern ideas.

• There was a tendency for colonist-era Europeans to ascribe exotic virtues to Near and Far Easterners that had little relationship to the values of those communities. Orientalism is a discussion of this dynamic related to Near Eastern culture. I don't think the dynamic can be well explained by reference to in-group/out-group, but post-modernism does a good job, in my view. Consider also the phenomena of the Magical Negro (warning: TVtropes)

• Death of the Author (TVtropes), the view that the author's opinions do not control a work's interpretations, is also heavily influenced by post-modern thought (or so I understand - I'm not very interested in most lit crit of any flavor)

• The slogan "The personal is political" is insightful because it highlights that "political" (i.e. partisan electioneering) is not really a natural kind in political-theory conceptspace. Issues of personal identity are just as mindkilling, for essentially the same reasons. Also, post-modern theory helps explain why the legal distinction between public action and private action is not well defined in practice.

Also, post-modernism is often intentionally filled with hyperbole. For example, I'm persuaded that Nietzsche was not anti-semetic or fascist, but reading Geneology of Morals literally can easily leave that impression. There are reasonable methodological arguments about whether hyperbole is a good idea, but post-modernism is usually on the side of more hyperbole (in part because colloquial usage often does not encompass a natural kind).

Finally, post-modernism is closely clustered with anti-capitalism and anti-empiricism. I can't defend this association, but it exists. I think much of the perceived poor quality of post-modern thought is really disagreement with those other positions. I don't think those positions are an essential part of post-modern thought - for example, I think Foucault is trying to be a high quality historian. If I were persuaded that his history was bad, that would necessarily cast doubt on his conclusions from the historical evidence.