Basically all other identity providers are also vulnerable to phishers which strip out webautn calls in the payload when acting as a proxy to the real IdP.

Basically you must disable all other phishable forms of MFA fallback if you want phishing-resistant FIDO2/passkeys. Conditional access policies in Entra can do this selectively or org-wide. If you don’t do this you’re relying on “end user training and wariness” again as phishing protection.