I think that both epistemic bubbles and epistemic bunkers on social media are more a function of the self selection of whom to follow, rather than the underlying technology, @SFuntowicz .

We might compare to traditional newspapers. A regular reader is shaped by the columnists he or she reads day to day.

The federated architecture of Mastodon allows for more interest-based groups, but in practice, I find it's not the local timeline that makes the difference. It's the moderators of that instance who allow or disallow, much as there are choices about who gets published in 'letters to the editor". Boosts to a post are by followers, not by moderators, though.