People are answering this wrong and focused too much on the site needing to have the same functionality as HN. The spirit of the question is where do other professionals (musicians, doctors, real estate developers, lawyers, etc.) congregate online. I don't know the answer for most of those but here are two examples
Not really the same spirit, but Marginal Revolution (https://marginalrevolution.com/) also has its own community around it and is one of the most influential blogs in the world. Also has an active comment section with an interesting cast of characters.
- SDN for aspiring doctors and med students https://forums.studentdoctor.net/forums/
- Wall Street Oasis for people in finance https://www.wallstreetoasis.com/forum
Not really the same spirit, but Marginal Revolution (https://marginalrevolution.com/) also has its own community around it and is one of the most influential blogs in the world. Also has an active comment section with an interesting cast of characters.