Whether a redpill believes it to be possible or not is irrelevant and the movies, in fact, counter any possibility that it may be possible with the attempted destruction of the Neb. "We proceed as planned?" The agent says to Smith, indicating that their intention was always to have Sentinels attack the Neb. regardless of whether Cypher was succesful, and any proposed deal.
A bluepill believes the noodles he is eating are "real" and made of a physical substance, that is not true (without getting into the philosophical arguements) anymore than the redpills belief that reinsertion is a real possibility.
And the Animatrix episode is a test, in fact the other redpill states something along the lines of "It isn't possible" if I recall, suggesting that most believe it is not, but there are some, who wish to return, that believe in it. Just as some believe the prophecy, some do not.
