I certainly would also agree with you that there is a malignant, anti-real, anti-science bent to Austrian school accounts of value for that reason (and also praxeology, don't forget praxeology).

But I guess my thing is that a lot of postmodernism philosophy is not about attacking objectivity or anything like that, but commenting on how western society historically and especially in modernity has oriented itself in an idealized way that is (deliberately?) unmoored from material causality. Nowhere is this more apparent than with Baudrillard's Four Stages of Simulacra, describing just that process of social unmooring (Baudrillard said that he wasn't a postmodernist, but Sartre said that he wasn't an existentialist (and Marx said that he wasn't a Marxist /j)), which reminds me a great deal of Debord's idea of the Spectacle. Derrida, to pick another example, did of course criticize people overestimating their direct access to the truth, but his response to that wasn't to throw his hands in the air and say it's all subjective, but to try to pioneer a process in literary analysis for understanding what a work really means ("deconstruction"). I don't remember if it was Marx or Engels (normally I'd guess Engels but I have the vague memory it was Marx), but one of them said that if objective truth was readily apparent to the senses, there would be no need for science, somewhat echoing arguments from people like Hume about how we cannot observe causality, only correlation, and so we need to base scientific models on inductive reasoning.

And of course, some of the point of arguments made by postmodernists like Deleuze are that these sorts of distortions are sometimes older than writing, as in his criticisms of "representational thinking" as an idealist distortion preventing us from understanding what something is by only thinking about it as an exemplar of an abstract ideal, rather than taking it for what it is and how it may extend past the potentials of the aforementioned exemplar that we might otherwise judge as strictly superior to it.

Obviously we can also rattle off criticisms for any of the thinkers listed, Marxists included, but my point is not that they are all shining beacons but that they have serious and worthwhile things to discuss.

I promise you that I have just as much contempt for anti-reality thinking as you seem to, and it is both a real problem and something present in what gets called postmodernism, but I can say that there's a lot more in "postmodernism" than just that, and I have only the most glancing familiarity with the tradition.