Could someone explain the criticism of BlueSky to me, i.e. that it's less decentralised? Isn't the AT Protocol set up in such a way that you could build your own social network using it, meaning you can interact with BlueSky users without being one yourself?

As I understand it, the critical flaw with Mastodon and Lemmy etc is that if I delete something on my profile it isn't guaranteed to be deleted on other servers - that is unfortunately unacceptable in the long run for most people. And account portability isn't clean, and you are at the whim of your server operator. How is it better?