To be fair, with ActivityPub your account is always tied to a given server.

However as you have rightly noticed, the person you are responding to here is being disingenuous. Controlling the account doesn't fix the fundamental issue caused by network effects. An account that can't interact with a large portion of the network is no different than Mastoson in practice.

The Mastodon equivalent would be running your own single user instance which gets defederated by one of the big players.