The author kinds of glosses over one of Popper's key insights in The Open Society and Its Enemies: the paradox of tolerance. He noted the danger of unlimited tolerance, and that those who are intolerant can exploit the tolerance of the the society to overthrow the open society.
We see this today in those who attempt to weaponize the idea of censorship when society attempts to minimize the destructive effects of intolerant views.
Further reading
https://www.stuff.co.nz/national/christchurch-shooting/11139...