Trump is correct constitutionally.
AG is top law enforcement officer. The fact that Trump hires does not mean he is the top law enforcement officer.
The Andrew Jackson Jewel Saga
“If it should be said that, the District Attorney having the power to discontinue the prosecution, there is no necessity for inferring a right in the President to direct him to exercise it—I answer that the direction of the President is not required to communicate any new authority to the District Attorney, but to direct him in the execution of a power he is admitted to possess. The most valuable and proper measure may often be for the President to order the District Attorney to discontinue prosecution.
The District Attorney might refuse to obey the President’s order; and if he did refuse, the prosecution, while he remained in office, would still go on; because the President himself could give no order to the court or to the clerk to make any particular entry. He could only act through his subordinate officer, the District Attorney, who is responsible to him and who holds his office at his pleasure. And if that officer still continues a prosecution which the President is satisfied ought not to continue, the removal of the disobedient officer and the substitution of one more worthy in his place would enable the President through him faithfully to execute the law. And it is for this among other reasons that the power of removing the District Attorney resides in the President.’’
B Wyman The Principles of the Administrative Law Governing the Relations of Public Officers