The "incomplete distribution" architecture of the Fediverse, in a sense, makes the filter bubble even worse than centralized social sites. A while ago, I saw a discussion thread on how increased speech suppression in China is driving some users to Fedi. One poster replied that he almost never saw any Fedi user from China despite using it for years, and found the claim hard to believable.

Meanwhile it's a matter of fact that the number of active users from China currently exceeds 20,000 on Fedi. And this community already has diverse interests for its small size. You can't find most of them at joinmastodon.org's index and probably have never ever saw them in your timeline, but there are several large general-purpose sites, spin-off communities from pre-existing domestic social sites, and also small instances for niche topics and personal uses. Some examples of the smaller sites out there: a VA-11 Hall-A themed instance, an instance for Legend of the Galactic Heroes fans, and I know at least 3 different LGBTQ+ instances (I bet there are more, unknown to me).

As long as there's no interaction between instances (many don't even want interactions), for practical purposes they don't exist. No user profile is fetched, no hashtag is shared, no post is processed. The takeaway message is, there are multiple independent galaxies in the Fediverse, many are lightyears away and not connected by hyperspace transport. Every instance has its own event horizon. Don't assume the Fediverse from your own instance's point-of-view is complete and representative.