Memes

Monopoly means that only a small amount of agents get access to it.

That was precisely the case. Durruti's column especially found that any "decentralized" command structure breaks down quickly into interminable squabbles, and like any revolutionary army, had to institute a strict hierarchy and centralized control. Most but not all anarchist groups accepted this centralized control as a necessity.

I suggest reading the tyranny of structurelessness on why "anti-authoritarian" groupings always end up either destroying themselves, or in cults of personality due to their individualist rejection of rules, organization, and structures of authority.

Monopolize does not mean that the group is small, but that the access is exclusive. The group that monopolizes can be large, and extremely exclusionary towards other groups. Anarchists in Spain monopolized authority, ran labor camps, and built up hierarchical structures over time to suit the realities of the Spanish Civil War.