@metadaddy : I tried configuring the connected App and the Auth provider. Unfortunaltely, I get the "error=redirect_uri_mismatch&error_description=redirect_uri%20must%20match%20configuration" error. The callback URL of the connected App and the Auth Provider matches. And just for confirmation, I've created the Connected App and the Auth Provider in the same developer Org.
Also, I've used Communities as you have pointed it out instead of portals and hence the community is not linked in the Auth Provider as how the Portal is linked in the example.