A part of the 14th Amendment, which I just noticed, clearly points out that United States does not mean the 50 American states.
It says, “Neither the United States nor any state…“.
If United States meant the 50 states, they wouldn’t need to add ‘nor any state’. But as everyone knew at that time, United States means the territories and the federal government, but not the American states.
Here’s the full text of section four of the 14th Amendment:
The validity of the public debt of the United States, authorized by law, including debts incurred for payment of pensions and bounties for services in suppressing insurrection or rebellion, shall not be questioned. But neither the United States nor any state shall assume or pay any debt or obligation incurred in aid of insurrection or rebellion against the United States, or any claim for the loss or emancipation of any slave; but all such debts, obligations and claims shall be held illegal and void.
Here’s a diagram to show how United States is different from the 50 American States.