Dennis, what you are referring to is something I've just read in the book, "You Can Still Trust the Communists to Be Communists (Socialists and Progressives too) by Schwarz & Noebel.
On p. 22 under TRUTH, they write: "The Communists [Liberals/Progressives] invariably tell the "truth," but it is the Marxist-Leninist "truth." Those who believe that the Communists will lie in the interests of Communism are mistaken. In fact, it is not possible for a Communist to lie in the interests of Communism. By definition, if a statement is in the interests of Communism, it is the truth." Therefore, conservative thought, ideas, or arguments are completely unfamiliar to those on the left. You are completely correct in your assessment.