Written while in exile during World War II, the book is a forceful defense of liberal democracy against totalitarianism. Popper argues that philosophies which claim to have a definitive, utopian answer for society's problems—as seen in the historicism of Plato, Hegel, and Marx—are inherently dangerous and lead to closed, authoritarian regimes. His work was a direct response to the rise of Nazism and totalitarianism, aimed at understanding and undermining the intellectual roots of such movements to prevent their recurrence.
The Open Society and It's Enemies
The strongest indicator that it is flawed is in the title The Open Society and It's Enemies. Structurally the title is this: "Thesis, and the enemies of my thesis". It is already militant and ironically authoritarian in its first words. Disagreement with the thesis are labelled enemies. So, rather than an authoritarian leader not tolerating political opposition or opinions, we have a political doctrine not tolerating political opposition or opinion. It is authoritarian. This time the doctrine itself is the authoritarian.
The Open Society is strongly singular in grammar. The choice of An would have implied applying a set of guiding principles to many variations (nations/states and their constitutions). The is monolithic, singular.
Karl Popper was living in New Zealand when the book was published in 1945. He was born in Vienna, Austria, and later became a British citizen the same year the book was published, resuming Austrian nationality later in his life. The book was not written to behave within or legally serve under the Bill of Rights or the Constitution of the United States. In the world this document imagines, the Bill of Rights does not exist.
If I was trying to create a totalitarian regime stronger than Hitler, I can think of nothing more resilient than the distributed nature of this document's doctrine. It is totalitarian without an apex singular leader. It is far less killable than the Nazi regime.