If she had been "covert" under the definition of the law under which he was investigated Miller would have been indicted for that. The author of that very law said she was Plame was not covert.
And that was precisely, I think, why Fitzy repeatedly used the term "classified," because you can be on a "classified" assignment or have a "classified" job that is not "covert" (i.e., undercover). Again, the example is a Navy Captain assigned a "classified" mission of delivering documents to a foreign navy secretary. He is not "covert." He is still Captain Smith. But his mission is "classified," or secret. It is entirely possible that Plame may no longer (if ever) have been "covert" and yet working on a classified mission or in a classified job description that Libby or others were not free to reveal.