What is the joke in mentioning Diedrich Knickerbocker's name and why is it ironic given what Crayon says??

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.