The context doesn't really change anything. Foucault's claim is about objectivity and subjectivity, the "no true politics" is a straw man which no one is arguing. There are (various) processes by which we can evaluate political systems on a variety of levels and refine them and these systems require objective standards. Frank's statement on materialist views equating to political activism is a non-sequitor and true only via circular logic. Why wouldn't Fish, someone admittedly concerned about his image, denounce a line that left him open to ridicule. Derrida's comment was on Deconstruction, his opinions on Marx economic or historical beliefs are contextually irrelevant.

I have no interest in engaging in debate about Postmodernism. Been there done that and - like practically anyone who has let it leech their valuable time - been left with very little to show for it. From all the hundreds of thousands of words written in its name, Postmodernism generated a tiny smattering of interesting ideas that, if applied in limited fashion could help open new perspectives for analysis of complex issues. As such, it is not completely devoid of value. Its development as the intellectual basis for movements such as intersectionality and critical race theory have been nothing but a regressive descent into emotive, faith-based belief systems that, far from offering nothing of value, actually undermine and seek to destroy systems that do provide not simply value, but fundamentally vital elements of a stable and intellectually open society. In this sense its worth is similar to statements praising Mussolini for making the trains run on time.

No intent to be impolite but I won't reply any further as, again, I have no desire to further explore the well-trod subject matter.