Sure, but passkeys are meant to replace passwords, not SSO (although I won't complain if they do replace SSO).

Most services probably would not restrict you to just one model of authenticator, but it wouldn't surprise me at all to see a service require that the authenticator be backed by a secure element, in which case you could use a security key, a passkey, a TPM, et cetera, but not a password manager. I'd still take that over passwords, but I don't think everyone would.