> But this still breaks the login flow for a very common use case: how do I log in on a device that I don't own?

at least in Apple land, if you try to sign on on a device that you don’t own (let’s say a work laptop where you’re not signed in to your apple id) it’ll give you a QR code to scan with your iPhone and it’ll do faceID on your iphone then do some bluetooth handshake to use your passkey on the other device. I’m not sure if this is Apple exclusive or if android/windows/linux would be able to do the same