How can I use iPhone as a roaming authenticator for webauthn?
According to the project's issue system, this type of workflow is not supported by the standard (yet). It is not a specific problem with iPhone.
Please refer to this issue:
https://github.com/duo-labs/webauthn.io/issues/21
Note that a similar workflow recently became possible on some Android phones. However it is implemented via CaBLE, as mentioned in the above linked issue. CaBLE is cloud-assisted Bluetooth Low Energy - which is Google's own technology, and not part of the WebAuthn standard.