I've even seen a number of subreddits with no moderators/inactive moderators which were pretty good, even with a few thousand members.
I suppose might be that the forums you're discussing may be one of the ones created with the express selling point of having low/no moderation, while these other places where created as a place to have discussions first and foremost.
But I had the impression that you habe to ignore a swath of racism, sexism, homo/transphobia, and conspiracy theory to find them.
For instance, many posts here about geopolitics appear to me as xenophobic and full of false information (particularly when they're countries that I'm familiar with). But it's hard for people to see that, or even think anyone could legitimately feel that way, when it's considered by the in-group to simply be "common sense." Heavy handed moderation about a subject (rather than simple topicality like with Hacker News) can lead to the entrenchment of these in-groups, which can both increases the level of these comments and leads people to think of them as nothing more than common discourse. It also increases bifurcation, so you have competing "destroy all Pepsi" and "destroy all Coke" forums, both of whom think they're merely bring moderate common sense ideas forward to oppose the extremists on the other end.
Having said that, the forums I'm talking about seem to have a lower level of this in general, since they often focus on a particular niche. Similar to how the rules against political discussion let's Hacker News have better discussions than in general, but when political discussions do slip through then tend to be closer to typical internet discussions.