Because Woodward will testify that he was told about Vallery Plame by someone other than Libby long before anyone discused it by Libby. He may also testify that many other reporter knew about Plame before Libby knew about it. Woodward says he knew. With how many other reporters did woodward discuss it?
Libby says he learned it from the press... That is Russert. Russert says he did not know. Woodward has said all along that there is no case against Libby, Rove or anyone else in the White House and that the case will collapse when it goes to court.
How could Woodward know that? He could know it if he had proof that Russert knew about Plame could prove it. Either Russert or Libby is lying. If Woodward proves Russert is lying then there is big trouble at NBC and no conviction of Libby.
Fake but accurate lies given in testimoney could get Russert indicted.