If something has hundreds of "centralised" platforms owned and run by a diversity of different people and spread all over the world geographically, then that's "decentralised".
I can't really think of another way in which something could be decentralised.
With ActivityPub, there's nothing stopping you hosting a server literally just for yourself. It wouldn't get much more decentralised than that.
Considering the fediverse microblogging scene includes Threads, which claims to have hundreds of millions of active users, I'd say its death is greatly exaggerated.
Yes, I know a lot of Mastodon servers refuse to federate with Threads, and yes I know their active user figures are likely very different from what they claim. But at the end of the day, it's an ActivityPub microblogging platform with a considerable userbase and a very rich corporate backer.