That is an oversimplification from (your link) an anti-Marxist anarchist. I've got a lot of sympathy for anarchists, but that doesn't mean that he isn't oversimplifying to help prove his point.
Marxism could broadly be split into two elements- the revolutionary and reformist branches. Famously, Marx declared himself a reformist, saying of the politics of two prominent revolutionary Marxists (Guesde and Lafargue) that if their politics represented Marxism, that "...ce qu'il y a de certain c'est que moi, je ne suis pas Marxiste" ("...what is certain is that I am not a Marxist").
The anarchist view is that any form of government- however democratic- is a dictatorship. Marx's view of a world governed by and for the masses would be a "dictatorship of the masses" in anarchist thought. That does not make Marx an authoritarian by modern (capitalist) standards; his view of things would have chimed nicely with our modern view of democracy.
I've always thought anarchism was very blue sky thinking- I see the appeal of it, but I simply can't picture a world where it could actually happen and where it could remain in a stable and sustainable state. This view is somewhat reinforced by reality- there have been a few anarchist revolutions and communities over the years, and none of them have ever grown to a meaningful size or survived for more than a few years. I'd love to be proved wrong, though.