He must have. They don't allege that he denied the prior knowledge, therefore, he must haved admitted it. Also, he attempts to reconcile his prior knowledge and his statement that he learned it from reporters by saying he forgot he knew. It must be that he tried to 'splain hisself. Unfortunately for him, his explanation was lame.
Well, as for making it into the trial of Wilson, it has the additional advantage of being true and a more palatable explanation than the one the prosecutor leaves on the table, that he is simply sliming an administration critic because he is consumed with a Nixonian bunker mentality.
Well the threashold question, is did Libby go back to the grand jury and testify again? Did he at that time say his earlier testimony was inaccurate? If he did that, then we are facing the issue, as to whether to cleanse you need to admit you lied like a rug. Rove, by the way, did NOT to that.