Libby is only guilty of being inarticulate in his statements to investigators and the Grand Jury.
Libby knew that Wilson's wife was CIA. He also knew it was protected information. He got this information from official sources.
If you're someone who knows classified information, and Libby certainly knew a lot of it over the period of time he was doing his job, you are very aware that you must be extremely careful when speaking with people who are not authorized to know, especially when conversations often involve the very subject to which the classified information applies.
Unofficial and Official data compartmentalization
Different people may have different ways of dealing with this. It is not unreasonable to assume that one method would be to compartmentalize the data in your brain by pretending to yourself you don't know certain details. You 'forget' them so you can't accidentally confirm information to someone else. It doesn't mean you really 'forget' them, you just tell yourself you've forgotten them so they stay in your head and don't come out your mouth.
And this compartmentalization would be dependent on your source for the information. Data from official sources goes one place, data from unofficial sources goes in another. You neither confirm, nor deny, nor pass on to others data that you learned from an official source. You, in essence, 'forget' that information.
The first time Libby 'learned' this information
So this was what Libby was doing and he explained it horribly. He would be surprised at Russert's knowledge of Wilson's wife's employment by CIA because Libby didn't 'know' that information. It's not that he lied about not really really knowing it. He didn't know it in the context of his conversation with a person who was not authorized to know it. In essence, this was the first time Libby heard it...from an unofficial source.
Libby couldn't simply say 'I heard that too' like he did later with Cooper. Because this was the first time the unofficial gossip that 'Mrs. Wilson works for CIA' had reached him from an unofficial source so he could not honestly say he had already heard it because that would be crossing the barrier in his mind between official and unofficial sources.
Libby's testimony to investigators and the Grand Jury
Reading Libby's statements to investigators and the Grand Jury in that light shows that he did not lie to them, he simply did not explain his method for keeping classified data to himself thoroughly enough.
"I was very clear to say reporters are telling us that because in my mind I
still didn't know it as a fact. I thought I was – all I had was this information
that was coming in from the reporters."
And that information coming in from reporters was the only information he could acknowledge. That was the only fact he could know publically.
What Libby testified re Russert conversation:
". . . . And then he said, you know, did you know that this -- excuse me, did you know that Ambassador Wilson's wife works at the CIA? And I was a little taken aback by that. I remember being taken aback by it. And I said -- he may have said a little more but that was -- he said that. And I said, no, I don't know that. And I said, no, I don't know that intentionally because I didn't want him to take anything I was saying as in any way confirming what he said, because at that point in time I did not recall that I had ever known, and I thought this is something that he was telling me that I was first learning. "
Again, if Libby's method to guard himself from revealing classified info from official sources was that he had to forget what he knew so he wouldn't actually confirm classified info, then this is not saying he (Libby) didn't know. It is saying that in his mind he would not recall what he knew officially, he would purposefully block it.
Libby is guilty of being inarticulate!
All those conversations with reporters
As for the timing and identity of who first told Libby unofficially that 'Mrs. Wilson worked for the CIA' the testimony is murky. There's no way for sure to know from the testimony we've seen.
Russert claiming he never spoke about Wilson's wife is he said/she said. It's also possible Libby misremembered and he actually heard it first from Miller. Possible that Russert misremembered. Possible that Libby is confusing Russert with an as yet unnamed reporter who has never come forward. Lots of reasonable doubt here.
'I heard that too'
And for the Cooper conversation, late in the process, read what the indictment says Libby said about it and what Cooper said. They are essentially the same. I don't see a problem there. 'I heard that too' is not confirmation, doesn't reveal where Libby heard it, and gives no information as to its veracity or even whether Libby believes it. And by that time Libby had heard it from another reporter. And because Libby had heard it from other unofficial source(s) it is a true statement. Fitz is wrong here.
(Not to mention reporters are incorrect when they consider 'I heard that too' as a confirming source, like Novak did with Rove. It is not. May explain why a lot of nonsense is reported. 'But I got my two sources!')
The Miller conversation in June with some clandestine guy doesn't mean much. In July it's not clear from Miller's testimony what Libby said anyway. And nothing is really clear about what Judy said to Libby.
So what we're really left with is...
The basis for all the charges is that Libby misled the prosecutor by claiming he heard the information first from reporters, not from official sources.
But Libby did hear the (unofficial) information first from reporters. The official information was locked away.
Why was Libby so inarticulate? I surmise because of all his years of dealing with classified information, his method of compartmentalization of data is almost subconscious. It is a habit.
Note: Edited for clarity.