This is a good discussion of the postmodernism, or a limited form of that, as it has been twisted into an icon to trigger fear in the reactionary mind. As a couple of comments have mentioned much of the core of postmodernism is the continued evolution of structuralism (Claude Levi-Strauss and Michel Foucault) through critique and reaction. I could throw more names into this but that probably would not any value here.
I need to agree with another comment that you missed a good bit by looking at this from its art and architecture origins and ignore its anthropological, philosophical, and historiographical roles. Needless to say those are my areas and I was heavily influenced by all of this including Thomas Kuhn in the early ’70s while working in intellectual history, psycho-linguistics, and early cognitive science.
In essence, you are correct. The use of the term postmodernism is an effective straw man (or iconic scarecrow as I mentioned above) for reactionary people who are all that is left of Western Conservatism. Unfortunately they have only two reactions to the inevitability of cultural paradigmatic change, outrage and fear. They cycle through those at an ever faster rate and become increasingly incoherent. As a result Trump is an excellent representative for them. It’s all lies!
The essence of the post-structuralist and postmodernist discussion is a broader realization that there is no sacred authority and reality is objectively real only at the atomic level. This is the universe of Schrödinger’s cat. The arrival of this particular cat in jokes and on tshirts at the turn of the 21st century showed the success of postmodernism in creating a new, popular understanding of a very complex scientific paradigm dating back to the Copenhagen interpretation and its problems. It’s all about what we know and how we know it.
This is not a failure, or refusal, to deal with reality, that works fine everyday, but a failure and/or refusal to recognize that even physical absolutes have the nature of analogies. The only absolute rules must be determined and include their own indeterminacy. That doesn’t really have much effect on our lives but it does force us to remain consciously open to change and our own role in that as an active observer if nothing else.
And that is, as you identify, a blow to the foundations of the reactionary mind. They do not want that to exist but it does. I think that pretty well sums it up.