Russert said he had not HEARD about Plame at that point, and therefore could NOT have told him.
The defense presented NO evidence that Russert had heard Plame's name before that point.
If Libby had testified that he was sure Russert told him Plame's name, at least they would have that as evidence Russert must have known, but Libby didn't testify.
I'm not sure how you mount a case defending against perjury without actually testifying that you didn't lie. Without Libby, the defense case was basically that you should not believe any of the people who are saying things different than what Libby TOLD the grand jury -- without ever presenting evidence that Libby was standing by his statements to the grand jury.