For more precision, the term "libertarian" (libertaire) was popularized in French language following the Lois scélérates outlawing any anarchist propaganda (1890s) under the 3rd republic [0] at a time when a few anarchists were openly murdering some of our overlords for their crimes against the people and such practice had widespread popular support (except in the bourgeoisie of course). There's even entire songs dedicated to the dynamite as a tool of emancipation against our masters, and to popular heroes like Ravachol (who bombed corrupt judges) and Emile Henri (who accidently, that's the funny part, bombed the commissariat in La rue des bons enfants).

So yes in the english-speaking world since the 50s/60s with Hayek and Randt) "libertarian" started to take a capitalist meaning. But it's worth noting that this doesn't expand to other parts of the world: in french-speaking cultures the capitalist libertarian movement is almost non-existent, and we use the distinct "libertarien-ne" word borrowed back from english for when we actually need to the american movement: "libertaire" certainly refers to the anti-authoritarian anticapitalist movement to this day.

[0] Yeah if you believe France is the country of human rights, that's just a myth. The 3rd republic had the same slogan (Liberté, Égalité, Fraternité) and flag (the Paris red & blue holding tight the royalist white) than the 5th, yet had widespread slavery and apartheid (following the "code de l'Indigénat") and women had no civil rights. French public schools was famously established in the 1880s as a means to expand the French colonial empire's reach. Don't even get me started on how broken/racist the 5th republic (since De Gaulle) is! :)

Which has even less to do with communism though.

On the contrary, both in anarchist and marxist philosophy communism is defined as a society without State and social classes. While anarchists believe (and practice!) in dismantling the established social order here and now by systematically questioning power structures, marxist-leninists promoted the idea that building a "dictatorship of the proletariat" was a necessary step for a revolution to seize the repression means of the bourgeois State to make sure to go towards communism. This disagreement is what led to the fork of the 1st Internationale in the late 19th century.

As history has shown, reestablishing the old forms of power (centralized State, political police, prisons, work) for the sake of the revolution didn't help to achieve any communism ever (USSR was profoundly anti-communist beyond the appearances: the secret police fought against all self-organized communes like Cronstadt or the peasant soviets of Ukraine). That's why anarchists to this day refer to marxism-leninism (what capitalists call "communism") as State-capitalism: it has the same structures and power balance than capitalism, with the difference that power is held not by the bourgeoisie but by a so-called avant-garde. But for the common people, the situation is very similar, as the promised Stateless/classless society is nowhere to be seen.

Marx eventually clarified that he thought the “dictatorship of the proletariat” would not be a dictatorship, but instead rather radical democracy. With the benefit of hindsight, we can say that the anarchists’ criticism of Marx and his dictatorship of the proletariat were spot on, and that such structures rapidly became authoritarian and self serving.

Not really, Anarcho-Communism, while not Marxist, is communist insofar as it is one of the remaining strains of "Idealistic Communism" as Marx and Engels would call the various worker's movements from before Marx' Scientific Materialism.

Kropotkin described himself as an anarchist communist in the 1890’s. There is a lot of historical meaning in the combination of those terms.

Communism != Marxism.

In principle i agree, yet there is a distinction to make. There are anti-authoritarian marxists who argue that by the end of his life Marx himself had become critical of his own theories (eg. dictatorship of the proletariat).

But yes, we'll agree that the most widely-known brand of marxism is in fact the very opposite of communism (as defined by a Stateless/classless society).