Postmodernism teaches (among many things) that truth is not objective. Rather, the epistemological center in postmodernism IS the self. Each self has its own version of truth, rather than truth being perspective-independent.

In literature, for example, this means that authorial intent is supplanted by perspectivalism. That is, each reader interprets a text based on his or her own perspective, and each interpretation is equally valid. This plays out in the political realm most prominently in the supreme court's interpretation of laws (does it matter what the author meant to say or just what we now interpret them to have said?).

So when I say it's impossible for the humanities to impose strict tests on veracity, what I mean is that a strict test would yield true results that are perspective-independent. This is what the scientific method attempts to accomplish, and the hard sciences use this methodology. The humanities, however, have long ceded that it is impossible to arrive at a perspective-independent conclusion and IMO would therefore reject a similarly rigorous approach.

I can only guess but art is now considered to be inherently subjective. So you there's no way to make statements like "picasso is better than jackson pollock", or "cubism is not real art".

Going further there are a number of works meant to probe the exact limit of what we're willing to consider art. I recall a couple interesting ones:

1. An artist took a shit in a can and sealed it up: https://en.wikipedia.org/wiki/Artist%27s_Shit

2. I recall there was an artist who took an existing book and changed the author name to their own, I can't find a link right now though.