Surprised that he doesn't mention Reddit, where the lost communities he described certainly do exist and thrive.
Reddit's a little too open to have that kind of connection.
However, being attached to the larger reddit "network" can easily ruin those communities. All it takes is a single front page r/AskReddit thread where a top comment says "check out r/[small_community_here], it's great!" and that pretty much spells the end of that small, tight-knit community.
It's the age old problem that's not at all unique to online communities: everyone wants to be part of the cool, fun communities/bars/neighborhoods. But if everyone crowds in, that community/bar/neighborhood isn't as cool or fun anymore.
I can see "the next messenger app" plus "total privacy à la signal" capturing that feeling back.