The problem is that anarchism proposes decentralized, communalist cells, making it impossible to actually practice total global collectivization along a common scientific plan. That's what Marxist communism actually is, a fully collectivized world economy, while the decentralized nature of anarchism is in some respects totally opposite. They share in common statelessness, classlessness, etc. only when judged from how Marxists and anarchists individually recognize class and the state, ie for Marxists anarchism retains class, while for anarchists Marxism retains the state.