The insistence on the primacy of narratives and interpretations does not involve a deriding of facts but an alternative story of their emergence. Postmodernism sets itself against the notion of facts just lying there discrete and independent, and waiting to be described. Instead it argues that fact is the achievement of argument and debate, not a pre-existing entity by whose measure argument can be assessed. Arguments come first; when they are successful, facts follow — at least for a while, until a new round of arguments replaces them with a new set of facts.
This is far from the picture of Nietzschean nihilism that Hanson and others paint. Friction, not free invention, is the heart of the process: You commit yourself to the standards of evidence long in place in the conversation you enter, and then you maneuver as best you can within the guidelines of those standards.