I understand how passkeys work. You don't need the legitimate app's credentials, we're talking about phishing attacks, you're trying to bring the victim to giving you access/control to their account without them realizing that that's what is happening.

A simple scenario adapted from the one given in the android blog post: the attacker calls the victim and convinces them that their banking account is compromised, and they need to act now to secure it. The scammer tells the victim, that their account got compromised because they're using and outdated version of the banking app that's no longer suppported. He then walks them through "updating" their app, effectively going through the "new device" workflow - except the new device is the same as the old one, just with the backdoored app.

You can prevent this with attestation of course, essentially giving the bank's backend the ability to verify that the credentials are actually tied to their app, and not some backdoored version. But now you have a "blessed" key that's in the hands of Google or Apple or whomever, and everyone who wants to run other operating systems or even just patched versions of official apps is out of luck.