Our prisonship of words is far from being a secure habitation. It is continually having holes torn in its sides, allowing the cold winds and seas of nothingness to sweep in. The second corollary of postmodernist antifoundationalism concerns power. If language cannot tell us of the truth, or be used to search for the truth, then philosophy, the love of wisdom, dies. It is replaced by rhetoric.
All relationships become power relationships of one kind or another.
In that situation, authority is inconceivable. If all speech is rhetorical, there can be no authoritative speech. There can be no relationships based on mutual recognition of the truth. Diggins does not put things in quite this way, but I think something of this sort is what he has in mind. Diggins' principal subject, however, is not the antifoundationalism of present-day European philosophy.