In short, Calcutt blames critial theory for the current state of affairs, via Lyotard’s The Postmodern Condition. Whereas I have yet to read that book I have read quotes from it and feel that I can confidently say that it was critical and not prescriptive! The author makes no effort to explain how they conflate observation of this trend in academia with their supposed “enabling” of it in political movements to harness populist sentiment such as can be seen with Brexit and Trump. So I pretty much write if off as another version of “blame the cultural Marxists” - although I give them a little credit for updating their stereotypes of the left by several decades.
One thing that many here even have struggled with is that Truth functions both as a philosophical concept as well as a common-sense one. I think it was largely the split of natural sciences away from the rest of philosophy which popularized the deprecation of truth in favor of evidence. Like the difference between human knowledge and mere data. All throughout the industrial age, technocrats have increasingly hidden behind data to create the illusion of political objectivity, of appearing to react pragmatically to a market-driven non-ideological landscape. But I think that the increased interconnections between people and quickening of communications technologies have simply made this illusion more plain for the average person to see. So as the masses see the lie, this revelation is coopted by the same interests who perpetrated that lie in the first place.
But a crucial difference I think is that post-truth in no way implies post-evidence. The notion that there is too much data, and no socially-cohesive models to expect its framing to the public is incompatible with liberal democracy. So we see a radical push for other forms of more centralized control, and open acceptance of disinformation and misinformation in broadcast media to normalize it in public discourse. The way out of it is probably to use media which are not reliant upon the structures of market capitalism. To avoid “truth” being treated as a commodity which naturally resides with those who possess the most capital - ie government and corporate interests. And to know the difference between claims of truth with being able to produce verifiable evidence.
ETA: my usual just waking up, pre-tea coherency disclaimer