Libby telling Miller or other reporters could be a crime. Lying about whether you did or did not tell somebody is a crime.
Miller did not say that Libby told her. Miller lying would be a crime.
If there is a crime let there be a perjury trial of Libby, at trial there will be witnesses for the prosecution and witnesses for the Defense. In a Libby trial who would Fitz call as a witness for his side Miller? Wilson? Cheney? Who would Libby have as a witness?
One thing prosecutors hate more than not finding wrongdoing is prosecuting and losing.