Authentication is, explicitly, the process of validating that you are who you say you are. Like biometrics, your username is part of your digital identity. So you are correct in arguing that biometrics alone is little stronger than a username, but by definition, both are part of authentication.
That said, to securely authenticate your identity, you need to use multiple factors.
I’d argue that biometrics is NOT an authentication factor but is instead a username. You’re identifying who you are.
Authentication is, explicitly, the process of validating that you are who you say you are. Like biometrics, your username is part of your digital identity. So you are correct in arguing that biometrics alone is little stronger than a username, but by definition, both are part of authentication.
That said, to securely authenticate your identity, you need to use multiple factors.