US government websites speak in such generalities that they end up misleading people. They tend to dumb things down so much that most people are mislead away from knowing what is all lawfully possible -- actually to most US citizens.

Most US citizens can lawfully go to Canada or Mexico without a US passport and return to the US without a US passport. [Unfortunately, the USG has required that the US and US-serving airlines require passports or passport substitution documents for air travel to those two countries too, but both of those countries still allow in US citizens without a passport even if the arrival is by air.