What are you having trouble with in this passage? It seems straightforward to me. Maybe if I restate it as the contrapositive it will be clearer: any rule imposed on someone without their consent reduces their agency and control over their lives, although if their agency was infringing on someone else's, that rule might increase that other person's agency by a greater amount. That seems so straightforward as to be almost tautological. You could maybe criticize it for speaking in abstractions that are open to misinterpretation instead of parables, but parables are notoriously imprecise, so your criticism would seem to be the opposite.

There's a little bit of conflict between the two sentences you quoted, in the sense that the second sentence doesn't explicitly take into account this possibility of one person's agency infringing on another's, but if there's an unstated assumption that imposing activity, as opposed to forbearance from activity, is never necessary to prevent that kind of interference, that would clean it up nicely.

The overarching theme in the passage you've quoted, as in the whole essay, is that there's an inherent tension between democracy and coercion; the employment of coercion endangers (Kevin's definition of) democracy at best, destroys it at worst.

Is that of any help? I'm kind of shooting in the dark here, since I don't understand the root of the difficulty, so it's possible that my explanations are just as opaque to you as the original text. Can you elaborate a little bit on what part you're having a hard time understanding?