The email and passcode is part of the site registration flow for this particular site (e.g. registration requires a verified email address). Other demo sites like webauthn.io do not attempt to add a 'real' registration flow.

My guess is that a demo app was pummeled a bit and fell behind on the email verification queue.

That aside, only the part where you register a new credential (e.g. the browser/OS modal dialog) in that registration process is using passkeys.