If you use a Yubikey as a passkey and the website does not require a PIN, then it is just something you have. Because if someone else has the YubiKey, they can access your account.
A roaming authenticator that is discoverable credential capable but not multi-factor capable can be used for single-factor authentication without a username, where the user is automatically identified by the user handle and possession of the credential private key is used as the only authentication factor. This can be useful in some situations, but makes the user particularly vulnerable to theft of the authenticator.