This seems to be a common problem, made more visible when using third-party authentication, that your application has taken the concepts of "Account" and "Authentication Method" as if they were the same thing. It appears that the "account ID", "preferred contact method+address" and "authentication ID" are all the same here - which then creates the "account management code into a rat’s nest" scenario they describe in…
I don’t see any technical reason this couldn’t be done, but it would be more work for both the app developers and the user.
Technically you can build an app that's purely a AR sticky notes specifically on your fridge... but the value of that app is approximately 0.