The key is to not use the fingerprint as a key for online authentication, we have a technique for that it is called cryptographic keys (either symmetric or asymmetric). Now people are generally bad at remembering these strong keys (and even worse at using them) so instead they use a trusted device (used to be a desktop computer but that day is past, now its a phone) to both store and use those keys. The user can then authenticate locally to their device using a less strong mechanism (traditionally passwords). Apple has this right, the device is the only thing that needs to use the fingerprint to authenticate the user (local authentication is by its nature two factor since you need the device). There is no advantage & clear disadvantages to using fingerprints directly for online authentication (passwords too as we have seen time and time again).