The US pays it’s bills. China defaulting devalues it’s currency, making payment necessary in dollars. The US has a hard limit on how much yuan it will exchange for dollars to prevent a run on the currency. China cannot get enough dollars to pay the bill, while the US is the one country in the world to which China can’t just say “lol no.” Assets will be forfeit or one sided deals favoring the US will come out of this. The devaluation of the primary currency of a major trade partner means their goods cost less and their interest increases
China needs buyers. If the US stopped buying from China, it would be the same situation in China. Their economies are so intertwined that a war between them, even just financially, would ruin both