How is that different from any follow-based network?

I think you're right that some microcommunities aren't exactly the greatest for your mental and social wellbeing. It's not hard to find websites today that showcase and bring out the worst in people. At the same time, I'd be tempted to suggest that the sort of thing isn't much different than someone starting to "run with the wrong crowd" in high school. Or, I think socializing on the internet reflects our humanity in ways that socializing in real life does too.

Decentralized microcommunities are just places to communicate with others, nothing about them is dangerous unless there is dangerous people in them. You can find all kinds of people in places like these, from left to right or even some pretty centrist folk. There are some pretty specific places on the internet where awful people live but I would say people are pretty capable of being radicalized on any platform including good old fashioned Facebook, Twitter or Reddit...

Decentralized microcommunities is how people interacted for the vast majority of existing.

Only recent, after settling down, feudalism, growing cities, and the last blip —globalization—, moved us towards centralization.

If anything, humans are great at managing small communities, and very bad at operating in a large, centralized, globalized world. Radicalism is often a product of that globalization, and would be unable to exist in a hunter-gatherer tribal system of small moving groups.

I'm not so sure about that line of reasoning; you're focusing on the size of the community (and sure, small communities online can be extremely dangerous!) but the nasty examples you're likely thinking of allow self-selection within a larger group. However, the kind of decentralization that follows local groups and families is different; you don't get to ignore that bigoted great-uncle (nor he get to ignore his crazy communist niece). If decentralization followed national or regional control as you'd need to satisfy conflicting courts, you're more likely to trend towards the latter kind of smaller community, not the former.

The size of the community is probably less relevant that how it is grown. Self-selection (like e.g. hackernews) is certainly much more vulnerable to groupthink.

good thing centralized authority and official truthmaking organs have no such track record i suppose