Like so many political philosophers, Michael Huemer fails to consider that all political authority occurs within a cultural context. Political authority isn't something that is created out of a void, nor is it something that people sit around and agree to; rather, political authority — the actual political authority that we are all witness to on a daily basis — evolves across history driven by processes so complicated even Francis Fukuyama's two massive volumes on the subject only do so much justice explaining.
Political authority cannot be removed from the context in which it is exercised. Nevertheless, Mr. Huemer divorces his remarks from any historical context by the repeated use of the word 'agent' whereas a proper noun would've been more fitting. One cannot understand Maximilien de Robespierre's authority in launching the Reign of Terror without understanding the context in which Robespierre's commands were carried out just as one cannot understand the legitimacy of Caesar's assassins without almost drowning in the historical detail of the end of the Roman Republic. They were all political decisions made whose decision makers, be they Robespierre or Brutus, thought that their actions carried along with them a just demand for obedience. Whether that was actually the case, though, cannot be examined from a purely formal point of view because obedience necessarily involves material considerations: Who is giving the command? What are they commanding?
Mr. Huemer mistakenly treats what he calls "the problem of political authority" from the point of view of everybody in society as equals, yet any historical point of view will recognize that certain people have the authority to commit acts that would have been viewed as unquestionably reprehensible if done by another. It did matter that Brutus and a band of senators, rather than Ioannes Doeus and his friends, stabbed Caesar to death. What separates an act of tyrannicide from an act of murder can actually just be the context.
Consequently Mr. Huemer's problem of political authority is only a problem because it doesn't open its eyes to the political reality that not everyone is equal in society. The state's decision makers can extract money from its subjects and not have it called theft because the state has a conventional authority to do such things whereas a private citizen does not have such an authority. Yes, the authority is conventional, but as Edmund Burke remarked in Reflections on the Revolution in France:
If society be the offspring of convention, that convention must be its law. That convention must limit and modify all the descriptions of constitution which are formed under it. Every sort of legislative, judicial, or executive power are its creatures. They can have no being in any other state of things; and how can any man claim, under the conventions of civil society, rights which do not so much as suppose its existence?
And yet, that's exactly what Mr. Huemer is assuming: Political authority without the social context that provide the causes of that authority. It's just a nonsensical way of actually understanding human society (and I hope that political philosophy actually aims to understand human society rather than build castles in the sky). It is an ancient principle that to understand something, we must understand its causes, and the same is true with political authority. To treat of political authority without treating of the conventions which give it force, as Mr. Huemer does, is simply the abuse of any metaphysical doctrine of right.
Perhaps those conventions which motivate political legitimacy are just bestial. Perhaps human society, by its very nature, isn't just, and isn't worthy of our better nature. St. Augustine came close to that statement when he compared Rome to a band of thieves in The City of God. St. Augustine recognized, though, that the vicious conventions, if I may speak of conventions as 'vicious', were a product of this world, and that to expect them to change for the better was naïve. Instead, St. Augustine argued that we should fix our eyes onto Heaven and care not for the political rubble around us.
Mr. Huemer offers no such consideration. If he wants to condemn modern human society — keeping in mind that the nation-state is part-and-parcel of our commercial society — as unbefitting to the human soul, then he is free to do so, but I doubt he'll follow St. Augustine's exhortation that the only society that matters is a spiritual one, unsoiled by the sordid realities of Homo sapiens politics.