Every serious software engineer that plans on developing public APIs at some point in their career needs to have an understanding of and some basic practice wit

Every serious software engineer that plans on developing public APIs at some point in their career needs to have an understanding of and some basic practice with implementing auth. "Don't reinvent the wheel" is easily bypassed by just reading auth code from prominent open-source projects to see real-world auth that is secure and works. You don't need a service and using a service doesn't negate an engineer from being competent to implement their own if necessary.