@mustaccio was correct.
The Docker Hub website allows you to login with either your username OR your email, and the website does not require a case-correct username.
docker login DOES require a case-correct username, and DOES NOT work with your email address.
When I signed up I chose a camel-cased username e.g.:
MyUsername
Docker forces this username to all lower case in practice. When you log in, you’ll see your correct username in the upper right-hand corner of the website. In this example:
myusername
The website allows you to login with MyUsername or myusername.
docker login only allows myusername.