....well, since nobody was able to provide any information, I researched again & found the solution.
https://community.keyoxide.org/d/12-support-indieauthrelmeauthoidc-login-to-the-forum/3
This is Flarum.
https://flarum.org/
Ironically, I have been dealing with Flarum a couple of years ago, so this discovery was awkward. 😆
I remembered it completely differently. It perhaps improved, over the years.
Case closed.