It seems pretty clear that Schlesinger is basically admitting the traditional understanding of the left right spectrum - with communism on the left and fascism on the right - and then proposing, however, that the two extremes actually bend back to be similar to one another.  This is very different from the multi-dimensional Nolan Chart style libertarian business.  I would also add that Schlesinger, as a historian of the United States, a country which has never seen viable communist or fascist movements, would seem a poor choice as a specialist to quote.  The point of Schlesinger is not to dispute that fascism is on the right wing of the classic political spectrum, but to suggest that the classic political spectrum itself is somewhat problematic due to these supposed similarities of communism and fascism.