It's not a question of Libby even recalling whether he had those conversations, it's whether he recalled what was then (June 03) a trivial sidebar to the real story about Joe Wilson's anonymous jihad in NY Times and WaPo...... it's that he claimed not to have heard about Valerie Pflame before July 03 and his conversation with Russert (interesting that Russert himself can't seem to "remember" anything at all except his certainty that he didn't tell Libby about Pflame). Remember that "Harriet the Spy" was a very small sidebar to the real issues until Novak and then the execrable David Corn published stuff about her. With Libby having dozens of conversations per day, it is in fact quite plausible that he "misunderestimated" when/how he first heard about Valerie the Spy.
I agree with you, but the jury never heard any real evidence to back that story up, since Libby did not testify.
The case presented to the jury was that Libby didn't lie because all these other people lied instead.
And you had to believe they were lying simply by how they were cross-examined, because the defense didn't present anybody who SAID they were lying -- only Libby could do that, and Libby did not testify.
Now, of course we have Libby under oath saying those other people lied -- but those were the statements that got him in trouble, and Libby never took the stand to STAND BY those statements.