I think the point is that being a "site for talking about everything" doesn't work. Ever. HN is restricted by what it is, even more than who runs it; a lot of the content here has no appeal to people outside of the tech world. The sort of people here tend to prefer rigor and statistics to emotional appeals and bloviating, and that creates a "house style" like old forums used to have. It's self-limiting, because there are only so many people who are, to be blunt, this nerdy in this particular way.
To take the Reddit example a sub like r/Labrats is self-contained and self-sustaining in the same way! There are only so many people with the life experiences in labs, wet or dry, and so many people who will get the humor and struggles of those communities. By contrast a sub like r/Worldnews is so vague and broad that it attracts tens of millions of people to talk about just about anything at all.
Quora falls into that latter category, "Ask me anything" in essence. I'm essentially calling for a return to niches based on limited shared interests or circumstances, because they come with their own ability to restrict growth and filter out "clots".
