this should have an answer, i think, so here goes.
as comments have noted, capitalism and communism are not opposite ends of a single line. especially not when communism means quite different things depending on who's talking about it, for example it can mean the authoritarian state/dictatorship of the proletariat (dicpro to its friends), which it has mostly meant in practice (so far), or it can mean a post revolutionary state of utopia (minor snark there), or it can mean any time any colleagues share something good with each other, etc.
capitalism is a particular way of being in the world, also varying widely from tendency to tendency: from any kind of exchange of goods at any scale of individual-to-group, to a hegemonic world view that requires expansion, alienates people from each other and the rest of the world, etc.
anarcho-communists, as far as i understand it, accepted some kind of communal vision of a post-revolutionary society, and differed from Communists (who also said they were aiming for a stateless society) primarily by refusing that the state could be a tool in the change to that society (that was the main theory difference. there were also differences in action, of course).
so, tl/dr: anarchists can reject a single world view as being the goal of our activity, which i would say is the easiest example of being against both communism and capitalism.