While those statements are not in the indictment, they will be brought into the trial, further mudding the already muddy waters.
It also helps in that Libby can claim that the perjury charges against him hold him to a higher standard of memory infallibility than all the other witnesses in the case.
In order to make that claim, he'd have to be able to demonstrate that other people not only made contradictory claims but did it under oath. Doing it in the press doesn't count for diddly. If he did make such a claim, I assume he'd have to demonstrate where others made similar contradictions under oath - which he couldn't do since the Grand Jury testimony would presumably still be sealed. Also you're assuming that the judge would let him try to even make the argument in the first place.