It's very much a Not Invented Here of Mastodon and the Fediverse.

Bluesky is a good user experience insofar as it's centralised.

Mastodon is a bad user experience insofar as you're forced to be aware of the decentralisation.

If you want successful decentralisation, Mastodon has that out of the box. You can stand up a Mastodon, Akkoma, GotoSocial etc on a $5/mo VM and you're an equal participant immediately. Or you can join someone else's server.

ActivityPub is underspecified and Mastodon just ignored a lot of it and so the actual protocol is an unholy mishmash of the two. It mostly works though, by the process of people beating on it until it works.

With Bluesky, you have a centralised service and a lot of people saying "decentralised!"

AT Proto is theoretically decentralised in the fabulous future and points of absolute and financial centralisation keep turning up.

I spend all day posting to both, fwiw. They each do a particular job. But the "decentralisation" in Bluesky is fake. Or at best, simply not feasiblly true.