One thing that always surprises me about these posts are that they always fail to get the arrow of causality right. In general postmodernist philosophy is a descriptive philosophy, not a proscription one. That is Foucault and the many other philosophers this author wants to write off as pseuds aren't stating "we should live in a world where all truth is in question" but observing that we do live in this world.
The rise of "astrology, homeopathy, flat-Earthism, anti-vaxx" in recent years for me is a strong argument that these thinkers were largely correct. People aren't reading Foucault and becoming anti-vaxx, and people who think Fox news is reality aren't doing so because they read some Deleuze.
While it is of course folly to try to paint all of these thinker with a broad brush, the general themes that they describe is that in our postmodern, late capitalist society we're seeing systems of meaning and value begin to break down. And again, they largely aren't saying that they should break down or that this is a desirable direction to go. Just observing that this is happening.
If these philosophies had made any predictions it's that as global capitalism grows more manic, we'll see the continued breakdown of systems of value and meaning, were people become completely enmeshed in a world of symbols detached for any real meaning. Not only the rise in "..., anti-vaxx" but also things like "fake news", the commodification of identity in social media, the surrealism of the radical "right" in the US, all of these things are the logical conclusion of what you would expect given the descriptions these thinkers have of who society works.
The bizarreness of rants against these thinkers is somehow blaming them for the very state of society, culture and knowledge that they predicted. It's like blaming epidemiologists for a pandemic. If anything we should view postmodernist thinkers as absolutely validated.