You’re not wrong, but there’s a nuance to it that you’re missing too. The Constitutional Amendment would be written to make the capitol district basically just the White House, the Capitol Buildings, and the Supreme Court, basically the size of the National Mall, and everything else around it would become the city of Washington, Douglas Commonwealth, or something like that. (The name comes from the DC statehood movement.) A bill to make DC a state passed in the House in ‘21, but never passed in the Senate, likely due to the question of Constitutionality. Conversely, the land that Maryland and Virginia ceded to DC could be ceded back to each state, and the residents in those portions would either become Marylanders or Virginians. But, DC will never, and can never become a state unless the Constitution is amended.