For sure, but that still isn't a passkey. The method you are talking about is the equivalent of non-passphrase protected SSH protocol, which is a single form of authentication (i.e. if someone has your security key they have your account).
The term passkey implies MFA: having a physical key and a password, a physical key and a fingerprint scan, or equivalent.
Sure the username could be considered the password, but usernames are not designed to be protected the same way. For example, they typically are stored in clear text in a services database, so one databreach and it's over.