The reason for the lie being that the AP continues to try and spin the Libby indictment as having to do with Valerie Plame's covert status (on which Fitzgerald has still not made a determination). The charges against Libby do no deal with Plame's covert status, so whether or not Libby was acting on the President's or Vice-President's orders to disclose Plame's employment is irrelevant to his defense. Only if Libby was claiming that he was told to lie to the grand jury would it be relevant.
Excellent point.