You often here pragmatists or pragmatist fellow travelers like Thomas Kuhn treated like relativists.
It's nonsense. And for some reason saying "no, I am not a relativist" for literally decades on end just isn't good enough (apparently they're self-hating relativists?). The framing of the third panel does a good job at separating two points - one where relativist postmodernists and pragmatists agree, and one where they disagree.
The boss concludes "...so you see, there is no right and wrong just an infinite number of equally valid 'stories'."
This isn't worded carefully to handle the traffic I'm about to burden it with, but you can think of pragmatists and relativists agreeing with the point "there is no right and wrong". Although pragmatists would probably say something like "if there is a right or wrong we have no sure access to whether we've hit on it or not". That's just a fallibilist claim that's not unique to relativists or pragmatists. But it is what pragmatists emphasize and it's what gets them called relativists. But it's not the only thing that pragmatists emphasize.
They also emphasize (and disagree with relativists on) the point that there is "just an infinite number of equally valid 'stories'." Now that is not true. These stories may be detached from reality, but they are not all equally valid. If you want to use the phrase "pragmatist theory of truth" (Rorty didn't like that, but we all know what it's referring to), that is precisely the approach to what is considered a "valid story" and what isn't. These stories are not equally valid. If you think that's what's being said then congratulations: you've missed the distinguishing feature of pragmatism.
I don't know if it's right to say that all post-modernists are relativists. I strongly suspect it's not true. Lot's of pragmatists consider themselves post-modernists, after all, and post-modernism is such a big tent it just seems very unlikely.
But if you're railing against leftists in the university it's probably convenient to pretend that they are all relativists and that the only way to avoid relativism is to avoid anything that smacks of post-modernism.