It certainly would diminish his motive to do so.
It would change the motive. If Plame was in fact covert, and Libby knew her status was "covert agent," then Libby doesn't want to be admit braking the "out the covert agent law."
But he may feel just as much urgency under any one or combination of motives I hypothecized in #14, above. In short, absence of "covert status" does not result in "absence of reason to lie to investigators."