A state is not necessary for enforcing contracts - that could be done by private parties, as in anarcho-capitalism. Also: Cooperatives can exist within a capitalist system, but the converse is not true. So I guess it comes down to whether or not all property as defined by the status quo should be redistributed or reallocated, presumably by force; if that were not the case, there wouldn't really be any disagreement between left-anarchists and anarcho-capitalists, right?