Diotima, I notice how the author, in making the outline (from the link) has underscored the point you are drawing attention to and one which Voegelin himself makes: "What is right by nature is identical with the best constitution, and its investigation is the search for the right order of society."
I think the key issue that Voegelin astutely points out is that "What is right by nature (or, the best constitution) is not given as an object about which one could state correct propositions once and for all."
People who take the U.S. Constitution as the be-all and end-all of law forget that it is rather a procedural document intended to be subject to what is naturally changeable. It is a constitution, as bb notes, that allows for this tension which Voegelin recognizes in Aristotle. It is not a living document, but it is subject to a living people. A constitution that allows for this tension is liable to derailment into the poles which constitute that tension.