While Mattheus seems to think social contract theory is the default position of progressive non-libertarian types, in a lot of ways its more of a libertarian approach. By that I mean that the problem that a lot of libertarians have with social contract theory is not the theory per se. The theory is great for their purposes. If we all explicitly or implicitly had a contract with the state, that would be just peachy. Their concern is only with the historical accuracy of the position. No such contract was ever struck, ergo no such legitimacy can be had.
In that sense at least, someone like Mattheus is more of a social contract theorist than I am. I just think it's a nice story with a good moral. He thinks there's substance to the logic of the theory itself.