Do you have any idea just how much property in a capitalist economy is possessed by someone other than the owner? What happens if I'm renting a house from you and you try to evict me? Now it's your protection agency--not mine--trying to take something that other people possess.
In fact, capitalist economies also have shades of legal ownership, i.e. lienholding--so I might even be the legal owner of a Camaro, but if I don't pay the car payment the repo man is legally entitled to take something I not only possess, but actually have some legal title to! And this only works because there's an unambiguous and authoritative legal framework in which these kinds of actions are permissible within bounds of regulation. OK, so if your anarcho-capitalist protection agency tries to repossess my Camaro and my anarcho-capitalist protection agency doesn't think your protection agency followed enough due process, where do we go from there?
The entire point of having a monopoly on the legitimate use of physical force is that these kinds of disputes are inevitably going to come up, competing sides are going to be unable to come to a settled agreement, and in the absence of a state, these disagreements will escalate to violence. This isn't a theory, it's the observed history of humanity.
If my characterization is incorrect, please describe a viewpoint-neutral algorithm capable of determining whether the anarcho-socialist group has a claim to the goods they do not possess. If no such algorithm exists, the socialist protection agency is indistinguishable from a group of theives.
It should also be noted that Huemer explicitly states that socialist groups would likely exist in the anarchy he describes. They would, however, not be able to coerce non-members.