Not necessarily. A future “Login with Discourse” service could operate like other social logins (Google, Apple, Facebook, etc.). Just because those logins are enabled, it doesn’t mean that the instance get access to all user names and email addresses at the source. The associated accounts will only be created for users that signup/login using that method.
The request in the OP is something that we have been considering for a while, it’s an interesting option that would reduce the barrier to joining a new Discourse community. But it’s a big, big project, and so far, we have hesitated to embark on it. Certainly an idea that comes up regularly internally.