It helps to read the full Appointments Clause, the last phrase of which states, “but the Congress may by Law vest the Appointment of such inferior Officers, as they think proper, in the President alone, in the Courts of Law, or in the Heads of Departments.” The term “inferior Officers,” read in the context of the clause, obviously refers to those officers who are inferior to the President and therefore includes the AG. Whitaker’s appointment being lawful pursuant to the Vacancies Act and that act being lawful under the quoted part of the Appointments Clause, he has the full power of the AG until a successor is appointed and confirmed.
The term "inferior Officers," read in the context of the clause, obviously refers to those officers who are inferior to the President and therefore includes the AG.
There would have been no need to include the term "inferior" if it referred to ALL officers appointed by the President. This has been the subject of a number of legal cases over the years, and there isn't a single one where that clause was interpreted the way you describe it. In general, the term "inferior Officers," in the context of that clause has been interpreted to mean any presidential appointee who does not report directly to the President. So a deputy secretary or assistant to a cabinet member would be considered an "inferior" officer. They may require Senate confirmation for other reasons (like the Deputy AG, who requires Senate confirmation under the 1966 statute that formally recognized the senior position of Deputy AG that had been established in the DOJ back in 1950), but not as a requirement under the U.S. Constitution.
P.S. -- Clarence Thomas weighed in on this very subject in his concurring opinion in the 2017 NLRB Supreme Court case.