One does not follow from the other. Your explanation would only make sense if the only two choices are keep cash or buy US debt.
Of course their holdings have to do with their belief in US debt, those dollars could be spent on many things.
Of course their holdings have to do with their belief in US debt, those dollars could be spent on many things.
Anything that they buy things that are denominated in $ are usually US imports hence goes back to the net export calculation.
Of course they can also use to buy global commodities like Gold, Oil which are denominated in $$, but that is just a proxy for selling $ in foreign exchange and using that currency.
So, logically only 2 options. Sell US $ in FX or buy US assets (Bonds, Stocks, Real Estate)