The passkey terminology is describing a "primary factor" user experience, which requires certain WebAuthn features to be implemented. A U2F-only key does not h

The passkey terminology is describing a "primary factor" user experience, which requires certain WebAuthn features to be implemented. A U2F-only key does not have storage so it cannot implement discoverability on its own. So it won't work as a passkey. A more modern Yubikey/security key will have the storage to do discoverability, as well as user verification (via pin or possibly via biometric) which some sites might also require.