I suspect Chomsky presumes an existence of the human condition for universal grammar. This in turn enables him to have confidence in his political beliefs by means of self reflection and knowledge of human history. In other words, I think Chomsky see's universal grammar (if correct) as a definite claim there is a human condition.
Of course, all this is mere speculation of my behalf based on the limited writings I've read. Am I being too speculative? Any references which prove or disprove this line of thought are welcome.