I think you missed their point. Yes, the specific beliefs held by the Reddit hivemind are specific to that platform. But the idea that Reddit has a hivemind is a natural human factor. So Reddit’s hivemind might be a centre-left liberal hivemind, HN’s might be more libertarian, and Lemmy’s is more leftist. But there’s some degree of hivemind on any platform that exposes users too each others’ content and where participating in those public discussions is the point.
A site like YouTube or Facebook lacks as much of a hivemind effect, because people aren’t on there for the discussion. They’re on YT for the videos, or on FB largely for their friends. Though both YT and FB comment sections are also proof that lacking a hivemind is also not a sign of quality.