Barring any evidence to the contrary, we can only assume that what Holder wrote is the truth. It certainly makes sense -- privileged executive communications, are, well, privileged, and each administration has a duty to defend that privilege. If you think his claim is false, or it should not apply in this case, then your remedy is the court system. If you don't like the concept of Executive Privilege, then your remedy is to lobby your legislators to introduce a constitutional amendment banning it.
If your remedy is to create partisan conspiracy theories, please be advised that I have no interest in them and will not address them.