1. Radical skepticism is false.
2. It is impossible to achieve a stable social order without reference to a common, objective reality that exists independent of the subjects who comprise the social order. If reality is taken to be always and everywhere just a phenomenon experienced by the subject, then all politics reduces to a struggle of wills, and the only order possible is the imposition of some wills upon others. Which is exactly what Foucault says, following Nietzsche. This can only produce a political order in which every dispute is adjudicated by ideologues.
Libertarians who go down this path are extremely foolish.