I think it is. Holder is a member of the District of Columbia Bar. Under Rule 3.3(a)(3) of the Rules of Professional Conduct for D.C., entitled "Candor to the Tribunal,"
A lawyer shall not knowingly:... Fail to disclose to the tribunal legal authority in the controlling jurisdiction not disclosed by opposing counsel and known to the lawyer to be dispositive of a question at issue and directly adverse to the position of the client.
In my opinion, failing to disclose the importance of the Espy decision to the analysis of executive privilege in this case does, in fact, violate this rule of ethics. Holder would thus be theoretically sanctionable.