Libertarianism never was what it used to be. It is a way station for intellectual young people to conservatism. You read “Atlas Shrugged” and for a few years you are a libertarian until you know a victim of the so called “victimless crimes” or try to figure out how to defend a nation within strict libertarian guidelines against a determined totalitarian enemy, and presto, a conservative is born.
The Founders, at least many of them, were personally conservative in behavior, but of (classical) liberal opinion about the role of the state. A sound position, IMHO. Remember, the Founder were almost all deeply versed in both English history (including Magna Carta, Henry VIII, Bloody Mary, Gloriana, the the debacle of the Stuarts and the Commonwealth, and the Glorious Revolution. The understood that liberty involved responsibility. Moreover, almost all of them were classically educated and deeply imbued with what were seen as the virtues of the classical Greeks and (especially) Romans. Many of them were deeply committed to the notions of 'civic virtue' - a willingness to voluntarily subsume one's own interests to those of the state for the greater good. Think of Horatio at the Bridge (in Macaulay's telling) or Cincinnatus at his plow (in Livy).