Yes I think this is the answer. Twitter at one point captured a type of user and part of the zeitgeist, when users earnestly wanted to meet others on the then small internet and were open to just having conversations. That internet was mostly nerds. Today's web (yes I changed the word from internet to web, as most people rarely use the internet outside of the web these days) is a very different experience. Today's web has everyone: it has your parents, it has your kids, and it has people from very different cultures and socioeconomic backgrounds.

The brief period when Twitter started was the start of a transition from a nerd-only internet to a general-use web. It was a single moment in time when everyone still felt open to internet socialization. Nowadays, niche interest and groups form insular groups the way they used to in the pre-internet days. The stuff I read on the more social HN threads could be interchangeably posted on IRC channels, Matrix rooms, or Mastodon posts. There's a more definite sense of "FOSS-y tech tribe" than I feel there used to be 15 years ago.

In that, it's just a return to form. Nerd groups were often niche groups in the past and now that everyone is on the web, they have returned to being niche groups. It is sad that we lost a brief moment of openness, when groups were interested in cross-pollinating. But it was just a moment in time, a mere blip in the development of social culture, rather than a lasting change.