The key here is Plame's status. There is every indication, though not proven by CIA documentation (at least the prosecutor claims that he has none), that Plame's identity was not classified, so there is no way that Libby could have official knowledge of her classified status. It's a simple matter of semantics.
Not in the sense of "covert or not." Did you read the indictment? What allegation in it (that matters to a count) is demonstrably false?