The difference here is you're allowing your phone's enclave to store the magic secret, instead of your human memory.
—If you're authenticating with a PIN instead of biometrics, it's still 2FA.
For example, if your 2fa codes for a service are always sent to an email account that uses the same password as the site for that service, then you do not actually have 2FA in that case since any potential attacker just needs the one password to get into your email account and that automatically gives them access to the other service.
If the passkey is stored on the device, and the device unlocks by bio-metrics only, and there is nothing additional but to tap the yes button on a notification in order to get logged in. Then the site or service you are logging in to, has only one factor authentication. You can also notice this in the fact that the system here does not send the raw bio-metric data to the service, only the passkey. Therefore if someone copies your passkey or finds a way to man-in-the-middle the key exchange, they can unlock the the service without having your phone. Again one factor.
For it to be a true 2-FA system, you would have to send the evidence of what you are and actually hand over what you have. Both would be checked, and only then would you be logged in.
In the case of the usual password plus auth app codes. The codes, and the keys the are generated, from are in the auth app. That's not ideal either, it can still be messed with, but that's why we pair it with something you know, which is not stored anywhere (if it is written down anywhere, even as uncovered text in a database, or a file on your device, then the password is not a password any more, it's just something else you have).