Even if they were to use a single cloud for the mabaged instances, this is not at all like the centralization of platform ownership. Here’s the critical difference.

If something happens to Twitter, say a methhead buys it and turns it into a propaganda machine, its users can only stop using it and/or move elsewhere. For this to have a significant effect, the whole network of people has to move. Every individual has to do non-trivial amount of labour to do so.

If something happens to the cloud provider hosting some sizeable Mastodon server, the owner of the server can migrate the instance to another cloud provider, or their own hardware, switch the DNS records and their users would only notice a brief interruptin. There’s no labour needed by the users. Only a much smaller amoubt of labour by the instance owner.

And that’s the critical difference.