A biometric describes what happened at issuance, not whether the result is independently verifiable. A photocopied passport is biometric-backed and proves nothing to a stranger; a DKIM-signed email carries no biometric and can be re-checked by anyone, forever. Making biometric a rung would rank the photocopy above the signature.
Keeping factors as a set also makes multifactor derived — two or more — rather than something a form can assert. And it lands on the federal definition exactly: a credential meeting the 8 September 2026 US verifiable-digital-credential definition is issuer-signed with factors device-bound and activation-factor.