On a related note, r-ChapoDogHouse is the last remaining chapo (in the name) sub that I know of. There was a porm themed one that lasted a while, but it got nuked eventually.
It's absolutely this, mostly because after only posting pretty much here for 4 years now I can smell the Redditor on people to the point where I am at like a 90% accuracy rate of knowing who is a new account or not. And I'm fine with giving them the benefit of the doubt for posting, but I don't like interacting with them too much.
Most of the online left is structured like a fandom or group of overlapping fandoms for compatible left media organizations and creators and academics. This is very cliquey as is most social media. Lemmy is like if individual cliques of nerds each had their own Hacker News (instead of the ycombinator group of nerds) with optional crossposting.