It's the usual map / territory or model / reality difference. If you've ever heard or said the Box-ism "All models are wrong, but some are useful" you've been introduced.

Some practice with it may even be a good intro to some veins of actual postmodernism (as opposed to piñata postmodernism which is mostly stuffed with straw by people who like hitting it).

This has nothing to do with postmodern philosophy. There is a difference between the reality, and a human's conceptualization of that reality. In fact, we can abstract further and say that there is a difference between X and the concept of X. Now, the GP comment is saying that the model of X (which is how a human being conceptualizes X) is different than X. So, when you look at nature as it is, there are no electrons. There are just things that humans happen to model as electrons. We could also happen to model them as something else perhaps inconceivable to us at the moment. But the reality of what-we-call-electron is independent from the concept of electron. This insight is as old as Plato and Kant.

I think GP's qiestion dates back to earlier than postmodern philosophy.