When saying things like "Unlike truly decentralized systems where users can participate equally regardless of their entry point" it helps to give examples of what those are, because thinking of my own, like E-Mail, confronts me with the fact that most of the issues raised also apply to it.

Server administrators also wield enormous power over their users. They can unilaterally decide to "defederate from other instances" in a sense (there's shared DNS blocklists that servers can and often adopt). Servers have content policies, they filter mail. Running your own server requires technical knowledge. Migration is a hassle.

There are differences, there aren't as many competing microblogging implementations of ActivityPub, but given ~92% of E-Mail servers are either Postfix or Exim that's basically a duopoly. I guess that's better? In any case, Mastodon is FOSS and anyone who wants to take it in their own direction can fork it.

In terms of comparing Bluesky and Mastodon's decentralized-ness, Mastodon is clearly more so. That doesn't necessarily make it the best microblogging platform, though.