Earlier quoted context omitted.
Category error. Federal departments are changes to the structure and function of government that should require Consitutional Amendments. The imbibing of Diet Coke by the POTUS is not substantial to the structure and function of government.
Where in the Constitution does it say that federal departments require Constitutional Amendments? As far as I am aware, the Constitution says nothing about either that or Diet Coke.
The powers not delegated to the United States by the Constitution, nor prohibited by it to the States, are reserved to the States respectively, or to the people.
I get the point you're trying to make that this seems to suggest that drinking Diet Coke is something also not "delegated to the United States by the Constitution" but I think people who disagree consider that drinking Diet Coke is not a "power" in the same way that operating the FBI is.