Berlin associated "positive" liberty with the Soviet Union, a country he fled. He also considered it the most easily perverted of the "liberties" - something abused by tyrants and people who think you are too stupid to understand that they know better (a paraphrase of his actual words). He was on the side of "negative" freedom with a touch of the "positive."

I doubt they even address this and just want to use the rhetorical impact of those words. Well, their readers probably won't question it.