I think this is using the Discourse discussion platform, which is open source: https://github.com/discourse/discourse
Well, now my statement makes a lot less sense, for some reason I thought this was the same as disqus. Thank you for pointing it out!
Well, the main community links are to the mailing lists: https://lists.racket-lang.org/
Understandably many people prefer Discourse to mailing lists though.
Discourse instances usually also support a mailing list mode.