It's not a "theory," it's simply reading Section 1 of Articles I, II, and III at a 6th grade reading level.
Article I says: "All legislative powers herein granted shall be vested in a Congress of the United States." May Congressional staff exercise legislative powers independently of the Congressmen? Nobody thinks that.
Article III says: "The judicial Power of the United States, shall be vested in one supreme Court, and in such inferior Courts as the Congress may from time to time ordain and establish." May judiciary branch staff exercise judicial powers independently of Supreme Court Justices and lower-court judges? Nobody thinks that.
Article I says: "The executive Power shall be vested in a President of the United States of America." May executive branch staff exercise executive powers independently of the President? My sixth grader could understand that the answer is "no."
There is nothing in here about the president being a "king." It's simply that the President controls the executive branch, in the same way the Congressmen control the legislative branch and the Supreme Court justices control the judicial branch.