Slavery is discussed in political philosophy all the time, even in "Anarchy, State, and Utopia" by Nozik (with which I happen to disagree, but that's nor here nor there). I did not read the first message as an hyperbole, but simply as a consequence of the fact that "enjoying freedoms while preventing others from enjoying them" brings slavery to mind quite often. It's an unfortunate, possibly unnecessary metaphor, but not necessarily a piece of rhetoric (although, like everything under the sun, it surely has been used to that effect as well).