Actually, it isnt so much that his legal reasoning is weak as much as it is that it is just wrong. When the US Supreme Court ruled that Special Counsels are not required to be Senate confirmed becasuse they are inferior officials that ended the legal debate, to the extent that there ever was one in the first place.
The Supreme Court case of Morrison v Olson ruled on the constitutionality of Independent Prosecutors appointed pursuant to a congressional statute, which provided specific statutory roles for the AG, the courts and congress with respect to the special counsel. That statute expired.
The Special Counsel on the other hand is a different legal animal entirely, created under no congressional statute but solely under DOJ regulations, and presents a different set of legal issues.