This seems true in 99% of cases, but it seems the rule is not quite as simple. "If you are an American citizen, you must enter the US on a US passport" is true

This seems true in 99% of cases, but it seems the rule is not quite as simple. "If you are an American citizen, you must enter the US on a US passport" is true for air travel. If you travel by land or sea, it is possible to enter with other documents (driver's licence, birth certificate... see https://help.cbp.gov/s/article/Article-3618?language=en_US https://help.cbp.gov/s/article/Article-3618?language=en_US). This is especially important in times of pandemic when the US government elects to stop processing passports and dual citizens may hold a passport of another country to be able to travel out, only to have re-entry problematic upon return...