It is Discourse’s own implementation of SSO. It is essentially the reverse of Setup DiscourseConnect - Official Single-Sign-On for Discourse (sso). If you’d like to look at some example code, the WordPress plugin’s SSO client code is here: wp-discourse/lib/sso-client at main · discourse/wp-discourse · GitHub. The trickiest part of implementing this on WordPress was dealing with the need for a unique nonce. Possibly that is something that will be easier to do with Drupal.
Any work that you do on Discourse/Drupal integration would be greatly appreciated!