Interesting — I wonder if 3rd party developers will ever get access to this functionality?
Specifically, nothing in this paper seems particularly novel from an engineering perspective.
Having said the above, I've always wondered why Apple doesn't have an iOS SDK for user-auth (or if they do, why isn't it more prevalent outside of the Game Center stuff). I'd much prefer to one-touch sign up for an app using FaceId (if needed/user-approved, the app could access user info, like email), then have to signup with an email/username or use another 3rd party like Google. This is especially perplexing to me after the addition of the iOS password manager (https://www.cnet.com/how-to/how-to-use-ios-11s-password-auto...) -- I think it'd be more compelling to offer password management behind the scenes in iOS (e.g, sign up with FaceId and have credentials generated/stored in password manager for off-device access; this could also encourage iOS generated passwords). There's a large chance I'm overlooking something here though, so who knows.