How do you know that "other sources" here means specifically sources unrelated to this case, and not "other sources" i.e., anyone other than Libby? Perhaps you have legal or factual knowledge unknown to me (wouldn't be hard!), but the plain English meaning of Bill Keller's phrase seems to me to say that Miller won't discuss any source except Libby. Please explain......
Simply because I don't see how a competent prosecutor or special counsel could make such an agreement as you interpret it and be properly discharging his responsibilities. On the other hand, concerns by witnesses to keep these special counsel investigations from becoming fishing expeditions are legitimate and a prosecutor could reasonably give the assurance in the sense I suggested without compromising the integrity of his investigation.