Anarcho-Communism isn't an oxymoron; people often (because Lenin) confuse Communism as an ideology with the practices of certain regimes which notionally were oriented toward achieving Communism (but never progressed beyond the phase they described as Socialism, which is distinct from Communism) and which leftist critics (including, among others, non-Leninist Marxists and other socialists) described as not even that, but State Capitalism.

In Marxist Communism (and, with variations that are important but not relevant to this discussion, it's derivatives through Leninism), "Communism" is the desired stateless end-state, and the "dictatorship of the proletariat" in which the workers seize control of the means of production through the State—Socialism—is a transitional stage used to get from Capitalism to Communism.

Non-Marxist Communism seeks the same stateless end goal by other means; Anarcho-Communism is, in a sense, among the purest forms of Communism, since it seeks fairly direct (not necessarily immediate, but constsntly in the same direction and not the out-and-back "empower the State to destroy the State" approach) transition to the statelessend end-state.