It is that there shall be holders of US debts.
Yes, US can print money to inflate away the debts. The end results is the loss of confidence on US dollar and citizen's saving's account evaporates.
That is equivalent to the bankruptcy. (although technically the country stay solvent but American people will be bankrupted through the printing process)
Currency collapse (or hyperinflation) is using countries savings to pay for the debts.
No difference, it is bankruptcy.