But this was not a use case that was using proof of real-world identity. How could that service possibly be relevant in any way? In fact, Github would know not

But this was not a use case that was using proof of real-world identity. How could that service possibly be relevant in any way? In fact, Github would know nothing about the identity of the Facebook account (and vice versa). This connection would be completely anonymous from both their perspectives. All Github could find out is is that somebody trying to recover the account had access to the preregistered (but unknown) Facebook account. And all Facebook would find out is that a user was recovering some unknown Github account.