Control is a sliding scale. They could run it on a physical box in Wikimedia headquarters and you could still argue they don't fully control the server because their ISP could always cut them off.
You have to make reasonable decisions based on your threat model and how easy it would be to move up the level-of-control ladder if needed. Getting on Mastodon at all represents a huge leap forward, and frees them up to migrate to a higher level of autonomy later.
Which is the point, the move to a higher level of autonomy is not going to happen later. It's far too much effort once they've already settled in.
We should praise organizations that actually seek to normalize control over servers, not praise relying on yet another "fully managed" service. We can do that while also recognizing that them being on fediverse is nice in general. All of these are possible without stating falsehoods like "They control the servers".
I don't know where in the world you are, but I want to live there. Changing my ISP is far more intimidating to me than migrating a database and a few DNS records.
> We should praise organizations that actually seek to normalize control over servers, not praise relying on yet another "fully managed" service.
What would be enough to satisfy you? A VPS on AWS? A VPS on a smaller provider? A dedicated box at Hetzner? Or would it have to be a machine that they built from scratch and can physically access?