For the record, we concur with article author Laura Hale Brockway's directive to stop what you're doing and see this movie (The Princess Bride) immediately if you have not previously.
That said, we have never been able to find what is incorrect in Vizzini's use of "inconcievable." Webster's give the definition as "impossible to comprehend." Contextually, it's just fine.
That nitpick does not, in any way, detract from our enjoyment of this story!