Did Libby run back to the grand jury to testify that he now remembered? I don't know one way or the other. Remember, it is a DC jury. Waging war on Wilson and the CIA as to what it was doing to the Bush adminstration may not be as appealing there as in say, Lubbock.
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.