Will Wilkinson, in a debate with Arnold Kling, says much the same thing:
Me: Fortunately, anarchocapitalism has little chance of getting beyond the thought-experiment stage.
A world in which I am bullied and coerced by lots of different people may be a world without monopoly, but that’s not a world of freedom. And Arnold is wrong that “the absence of monopoly means that you can exercise exit.” Suppose you’re in an anarchocapitalist world (a world in which we do not “take it as given that the political jurisdiction where I reside is a monopoly.”) You live in a house on a piece of property boxed in on all sides by other pieces of property. Each owner of an adjacent property has credibly committed to shooting you if you trespass on her land. There is no collusion between property-owners. They’re just independently jealous of their property rights. Here you have a situation where there is an absence of monopoly and an inability to exercise exit.