That's all true. But that's not really a string - it's just a fact of any for-profit organisation that sets up an instance
Correct, and it's a fact that's horrendously bad for an organization that's going to harbor a vast majority of the content on the fediverse.
But he can do that anyway. And in fact people who who want to interact with the 140million ish Threads users currently have one option - join Threads. With federation I can communicate with Threads users without joining Threads. That needs to be factored in.
The people who are at currently at this point have already gone to Threads. The main issue I see is everyone getting used to the 50x boost in activity that Threads provides and then Meta removing that by defederating, pulling people to Threads when they wouldn't have gone there otherwise. Allowing ourselves to become dependent on Meta lets them get users they wouldn't have before and kill the growth prospects of platforms like Mastodon, both of which they have incentive to do.