I would guess they're talking about niche gaming forums. I know a bunch myself - /r/Factorio, /r/KerbalSpaceProgram, /r/Subnautica, /r/StardewValley... oh wait, the latter two aren't that niche.
In my experience, it's a matter of focus and moderation. Unfocused groups turn quickly into toxic cesspools, whether the groups are made of nerds (e.g. 4chan) or genpop (Facebook threads, or comment sections in those publications who didn't yet turn them off).
Yes, you are right. I was wrong to being up plain “gaming”. I should have made that clear - the dwarf fortress forums are benign and healthy. The world of Warcraft forums are lexical hellscapes in comparison.
As long as the game has some effort barrier which needs to be overcomed to be enjoyed, the community will be healthy and civil. (All things held equal)
There are plenty of factors that play into how or why a forum turns to shit. You're right that size or "niche" isn't always the dominating factor, but in most cases, it's clearly important. There are, on the other hand, plenty of incredibly niche and focused communities that do turn toxic and insufferable (as any Darkfall game shows). People are complex and we tend to talk in short-hand because we don't have the time or desire to write entire papers about why people tend to suck when you put them together.
In my experience, it's a matter of focus and moderation. Unfocused groups turn quickly into toxic cesspools, whether the groups are made of nerds (e.g. 4chan) or genpop (Facebook threads, or comment sections in those publications who didn't yet turn them off).