To avoid default the US will fire up the printing presses. My guess is the Fed and Treasury are intertwined to provide as much money as needed to keep the world believing the broke system isn’t actually broken.
So whether it comes from the Treasury or Fed, it’s still going to be made up money.
The bigger fear is if other countries holding our treasuries devaule their own currency to inflate the value of their US$ denominated holdings, or worse they tire of our shenanigans and cash them out for another country’s debt to “invest” in, and the dollar loses it’s place at the world’s reserve currency.