Libertarian=social liberal on the political compass quiz, so a social conservative might find himself labeled an "authoritarian", even if he has no desire for the government to enforce his moral views. Still, you'll note that even by that flawed definition, he's less authoritarian than everyone except Gravel and Kucinich.
Eh? He is more authoritarian than the average libertarian. He's completely anti-illegal immigration, (an authoritarian stance) considers gay marriage "a perversion of liberty," is completely against abortion, and is against secularism. The fact that he wants to leave stuff to the states doesn't change the fact that those are not very libertarian stances. Besides, more importantly, he's much more libertarian compared to the rest of the candidates.
It's not flawed, either, since there really isn't that much difference in the political opinions of the candidates of the US - two party systems do that.