Anarchism is a metaphysical, moral theory which postulates ahistorical, acontextual "truths" such as: violence and domination are immoral, therefore we oppose them in all situations, etc etc etc. Many anarchists claim to be non-pacifists, but in practice they never support anything more than property destruction, and when confronted about property destruction ie in black blocs, they retreat into pacifist yammering.
When anarchists have broken from pacifism, they veer into individualistic terrorism. See the examples of Leon Czolgosz and the two anarchists who kneecapped an Italian nuclear energy CEO in the last few years. In both cases, these acts of terrorism, orchestrated by individuals, targeting individuals, are flashes in the pan that don't, in the long-term, accomplish anything. The two anarchists who kneecapped the CEO admitted as much when they were sentenced, although I wish I could find their sentencing statements again to provide an exact quote.
Meanwhile, multiple communist parties around the world are at this very moment engaged in protracted revolutionary wars to accomplish the goals of revolution, seizing land, expropriating capitalists, etc.
Marxism (and Leninism, and Maoism) are dialectical theories of practice. They posit historical, contextual truths, like: the freedom of the bourgeoisie is based on the oppression and exploitation of the workers, and therefore there is no way to free the working-class without in some way infringing on the freedom of the bourgeoisie. The freedom to buy and sell labor-power, for example, is nothing more than the freedom to exploit. The right of private property is the right of capitalists to deprive workers of the product of their labor. Thus, "authoritarian" methods are accepted as one part of the process of ending capitalism, because it is based on the recognition that moral categories are never neutral and "above history" but are always partial and embedded in history. There is no way to free the working-class without infringing on some people's "freedom" to exploit and the "right" to immiserate the majority of people in society.
Anarchism takes the moral categories inherited from bourgeois philosophy and ethics (such as the individual subject, possessor of rights and property, etc) as given, and tries to go beyond them while still basing itself on these categories. Marxism critiques these moral categories and points a way beyond them, through revolutionary practice.
I will freely acknowledge that I know plenty of anarchists who are not as narrow-minded and dogmatic as all this, but then again, when they fix up their perspective and their practice based on a historical materialist analysis, they can only do so by in practice breaking from that which is distinctively anarchist about their perspectives.