True, that is maybe too strong a phrase, but I think it's close to accurate. I think the culture & medium provide kind of a self-selecting gate: it's just plain text and links to articles, with the discussion expected by culture to be fairly serious. I think that turns off enough people that it kind of forms its own gate shutting out the people that make "eternal Septembers" happen. But yeah, ultimately, you're right.
The people who make Eternal Septembers happen show up here all the time. They show up here from Reddit and 4chan and elsewhere expecting this forum to be a free-for-all because the word "hacker" is in the title, and are inevitably shocked and disappointed to learn how aggressively moderated and tone-policed this place is.
They're the ones who post racist diatribes under green alt account and anti-vaxx conspiracy theories and complain about wokeness and talk out of their ass about subjects they have no actual expertise in. The ones who think flagging is just a super downvote.
I mean, the premise that simply having a minimalist layout would filter for a better class of people has always been a bit silly. Eternal September is an inevitable result of having new people join a community. Every new person, every new comment, increases entropy. It doesn't matter how technically proficient they are, they will change the culture simply by virtue of their existence and participation at scale.
I think that HN eternal september'd at the beginning of the pandemic and the Reddit Apollo stuff was the nail in the coffin.