No, I think the point is that Armitage being the leaker tends to show that Libby had no real motive to lie.

Armitage was anti-war, I believe. Therefore, he had no reason to leak Plame's name as revenge for Wilson's anti-war statements.

Since the leaker was not motivated by a desire for revenge and, in fact, probably released her name by accident, why would Libby have lied?

Libby claims his "perjury" was the result of faulty memory. That there seems to be no motive tends to confirm that. Which may create "reasonable doubt."

If there's reasonable doubt, there's no conviction.