That's true for domestic public debt. But in the scenario given by the parent where the dollar falls out of favor, it is assumed that we could be issuing public foreign debt in foreign currency. Even if it was still domestic currency, the FX rate would matter to the foreign investors. Interest rates matter. So does inflation. Money supply is less relevant than the actual inflation it generates. Most debt instruments rely on the interest rates that fluctuate based on monetary policy to combat inflation. Eg your interest on debt will increase as your inflation rate does. Even the world bank will jack up your interest if your currency has issues, such as rampant inflation.