"It merely implies he lied to Russert but does not necessarily mean he lied to the GJ."
Russert claims that he and Libby never spoke about Wilson's wife. Libby claims that Russert is the person who told him. Someone is obviously lying. In order to determine who is lying, the content of what Libby claims he told Russert is relevant. Since what Libby claims he told Russert is not factually true, it's an indication that Russert is the one who is telling the truth, and Libby is the one who is lying. I can't make it any more clear than that.
Libby can testify in court that he intentionally lied to Russert, and thus, the content of his conversation with Russert should not be an indication of who is telling the truth. It will be up to the jury to decide. The fact that what Libby claims he told Russert is not factually true will probably weigh against Libby in jury deliberations (if this case ever gets that far).