The author makes some good points, but imho undermines his argument by leading of with concepts like 'you come to believe in an extremist idea';
extremism is a word heavily freighted not only with meanings of intensity, inflexibility, or relative unpopularity but also with the implication of exceptional consequences that follow thereon, like rounding up perceived enemies of the state or overthrowing the social order. It might have been better to start from a perspective of unpopularity or rigid belief in a particular principle, which then hardens into an extremist position following the process of rejection and retrenchment the author describes.
On a side note: Suppose you become politically radicalized and become a Marxist. You get convinced that the labor theory of value is correct.
Marx did not believe in the Labor Theory of Value, although the above is a very common misconception because a lot of self-avowed Marxists do. On the contrary, Marx believed that some things had inherent value, like an oasis in the desert which provides a much-needed resource without any investment of labor. The LTV was adopted as part of the platform at the inaugural conference of the German Social Democratic Party in 1875. At the time, socialism was split between followers of Marx on one side, and followers of socialists like Ferdinand Lasalle on the other. The differences are too complex to delve into here, but you could say that Marxian socialism was more revolutionary and aspired to a general reorganization of society, while Lasallean socialism was more reformist and oriented around securing rights around pay and working conditions via collective bargaining, rather than having a general hostility to the economic order of the time.
Anyway, Marx did not attend this conference but when he was subsequently sent a copy of the program adopted there, he was so annoyed that he wrote a blistering critique, attacking the what he saw as the weak intellectual foundations of the plan and arguing that that they would fundamentally undermine the socialist project. You can read the cirique here, along with a much more comprehensive introduction that provides a lot of additional context: https://www.marxists.org/archive/marx/works/1875/gotha/index...
Marx's writing has a reputation for being turgid and difficult, which I think is somewhat undeserved. But lest people think think I'm just doing the academic or political trick of telling you what I think it means, here's the opening couple of paragraphs, in which Marx gets straight to the point:
First part of the paragraph: "Labor is the source of all wealth and all culture."
Labor is not the source of all wealth. Nature is just as much the source of use values (and it is surely of such that material wealth consists!) as labor, which itself is only the manifestation of a force of nature, human labor power. The above phrase is to be found in all children's primers and is correct insofar as it is implied that labor is performed with the appurtenant subjects and instruments. But a socialist program cannot allow such bourgeois phrases to pass over in silence the conditions that lone give them meaning. And insofar as man from the beginning behaves toward nature, the primary source of all instruments and subjects of labor, as an owner, treats her as belonging to him, his labor becomes the source of use values, therefore also of wealth. The bourgeois have very good grounds for falsely ascribing supernatural creative power to labor; since precisely from the fact that labor depends on nature it follows that the man who possesses no other property than his labor power must, in all conditions of society and culture, be the slave of other men who have made themselves the owners of the material conditions of labor. He can only work with their permission, hence live only with their permission.
The whole thing is fairly short, maybe 8 or 10 pages though obviously the prose of 150 years ago is rather dense; also Marx was writing to argue with an expert audience rather than to explain to a general one. Nevertheless, it's pretty readable and probably the clearest articulation of how Marx anticipated a 'dictatorship of the proletariat' would operate on the way to full Communism (where Marx imagined all economic productivity would be owned and operated by workers in general, and envisioned everyone working part-time for 20-30 hours a week and having abundant time left over for education, entertainment, time with family, or whatever they happened to enjoy).