It seems to get more confusing:
// Sets one variable to the username and one variable to the password given in the credentials.
usernamePassword(String userVariableName, String passwordVariableName, String credentials)
versus:
usernamePassword('PASSWORD', 'keystore password')
Makes for a rather confusing example. At least the example could say 'USERPASS' as the variable name, or show binding for both values, maybe. (And can credential id's have spaces?)