Peter McNight makes two serious errors. First, contrary to McNight, Enlightenment science does not purport to discover Truth. It accepts that there is an objective reality "out there" but also realizes that we can never fully discover it. We can only progressively debunk misconceptions about it. There is no Absolute Truth.  But, it is possible to detect and debunk Absolute BS, provided that people are willing to agree on the ground rules - e.g., assertions of fact must be evidence-based. 

Second, post-truth anti-Enlightenment thinking goes back beyond the left-leaning thinkers of the second half of the twentieth century to the idealist tradition German philosophy. That tradition gave rise to thinkers on both the right (e.g., Heidegger) and the left (e.g., Foucault).  So post-truth is not a new invention. The Nazis and the Communists were both enemies of the Enlightenment. McKnight himself engages in  a bit of post-Truth by blaming Trump on the left.