This is the same criticism that was made of cryptocurrency's claim to fame regarding decentralization, consensus, and resilience to authoritarian takeover.
"If you take all these different parts of your identity, all the games you play, all the things you buy, all the groups you join, and stick them into one system, that's a central system. It doesn't matter how many servers that system spans, you've pooled all that data in one place."
And ultimately we can make the same criticism of the Fediverse itself. It's nice that there are different platforms, different instances, different communities... but it's still just one entity at the end of the day. This is especially apparent with the spam wave we just saw. Misskey, Mastodon, Lemmy, even kbin was not invulnerable. You don't need to attack them individually, you can attack them all at once, and then they will naturally spread your attack to other instances for you.