Robin's standing there, attempting to be moonshine, does not make him so, even if he is holding a lantern to represent at least a part of the Moon. Similarly, Shakespeare seems to be arguing that no representation of anything in a play can really be completely real or truthful, no matter how hard its players may try. Rather than begging forgiveness of the viewer, he is exaggerating the problem for their entertainment.
Robin's attempt at being a moonshine figure does not make him one, even if he holds a lantern to represent the moon. Similarly, Shakespeare argues that no representation in a play can be entirely real or truthful, regardless of how hard the actors try. He is not seeking forgiveness from the audience but rather using exaggeration for their amusement.