Your HN example makes me think: if I'm talking to my spouse or close friends, we obviously don't have a moderation policy, we know each other well and share values well enough that any debates we may have are (almost always) focused on substance and not on conduct.
In political discourse, and in debates on big platforms like Twitter, it's the opposite- most of the discussion is about people's or groups' conduct and substance takes a back seat. Because a heterogeneous group with different values is involved.
So for social media and online forums, the question is, how big and diverse can the audience get while still supporting civil, substance focused discussion? HN does a pretty good job, and also has some obvious biases, scared cows, and weak spots. Online newspaper article comments probably have one of the lowest quality of discourse for a given participant size. What forum is best, I'm not sure, but its instructive to look at it this way, because it reflects politics generally, if we want to address real issues while maximizing participation.