I guess it's making a false statement to mislead the FBI about misleading Tim Russert when Fitz is misleading about Plame's employment at the CIA not being widely known.
Look, if Libby was lying to the investigators, he should have his hand smacked for that alone. Nonetheless, from listening to what Fitzgerald was saying, there are several logic gaps. Maybe things will match up when the case is made, but at this time, there are several instances where he is arguing that "if A notequal B; notA = B".
Further, aside from the possible lying, Wilson was attempting to misuse the law protecting CIA agents identities as a shield against uncovering his lies. It is a total and complete abuse of the law to even denounce someone as violating it for defending themselves against false accusations, and frankly, I question Fitzgerald's ethics for not making that clear.