The trick is, Holder could have known that there was an initiative called Operation Gunrunner. He might have known its broad parameters. However, if we're to allege that he knew (and approved) of a specific policy of allowing guns to "walk," we're going to need hard evidence.
A gunwalking policy was apparently approved by someone. Who would think he has the authority to make a decision like that one that affects foreign policy with a neighbor country?
My thought is, if I were Attorney General and someone came to me asking for approval for something like this, I would go to the President. Foreign policy is the President's job, not the AG's. I do not think Holder has the authority to approve this. Only Obama does, in my opinion.