Apparently, as interpreted by the Monarchist Party, the words
The validity of the public debt of the United States, authorized by law, . . . shall not be questioned.
Congress may not, by law, limit the amount of debt which is authorized.
Ergo, so the tale goes, the debt limit must be unconstitutional.
Just like the War Powers Act unconstitutionally restricts the President's power to declare war. Oh, wait....