The activist part of what you described comes from a 1965 paper by Herbert Marcuse called "Repressive Tolerance."[0]
The ideas from Marcuse's paper are widely mis-attributed to Karl Popper in that infographic everyone's seen floating around.[1]
[0] https://en.wikipedia.org/wiki/A_Critique_of_Pure_Tolerance
[1] https://www.libertarianism.org/sites/libertarianism.org/file...
Lindsay defends the old-academy classical liberalism. The sort of gentlemanly debate that arises from such values is good, but only possible between people with a collegial relationship who both prefer truth over power. They cannot be used against enemies. When dealing with enemies, one must bring weapons, and the postmodernist weapons can be made to work in anyone's hands.
https://graymirror.substack.com/p/a-brief-explanation-of-the... (An oligarchy inherently converges on ideas that justify the use of power.)
I think it's as good of an explanation as any for why old-academy classical liberalism is losing (and will continue to lose): truth is not dominant, and least on the timescales the game is played.
100% agreed that post-modernists are still worth reading, and that many of the critiques are relevant. Disagree that doing so will change anything—because truth isn't dominant in the short-term.