The joke in mentioning Diedrich Knickerbocker as the source for the story comes from a knowledge of other works of Irving. In 1809, Irving published his first book called A History of New York from the Beginning of the World to the End of the Dutch Dynasty under the pseudonym of Diedrich Knickerbocker. The story itself is outrageously satirical. Though this is done purposefully, the tone is accomplished by making the "author" (Knickerbocker) sound completely illogical and at many times, foolish.
The irony of the claim that "Rip Van Winkle" was found among the papers of Diedrich Knickerbocker is that the reference comes with the suggestion that its purpose is to validate the truth of the story. Anyone who had prior experience reading Knickerbocker, however, would immediately expect the same sort of humor and foolishness as was present in History of New York and naturally doubt the credibility of this story.