Communicating on a platform you don't own and can't control seems very shortsighted.

I feel like this would be a much more realistic take if social media more broadly was all federated, and anyone's independent instance could still communicate with the others, but that's unfortunately not the case.

For a politician, which is better for their campaign? Starting an independent platform they entirely own and control, but with no local users to start out with, or having an account on an existing platform with millions and millions of users?

Obviously, even though in the first example they would have 100% control over their infrastructure, they wouldn't exactly be spreading their message very far. They could always publish simultaneously on both platforms, but that still doesn't mean much if the second platform has no users. However, the platform that has many millions of users can instantly grant them reach, which is kind of the point of them being on social media in the first place.

On your point about a bot, I'm assuming you mean more like a bridge mechanism that cross-posts from one platform to another. You could correct me if I'm wrong, but I believe AOC at least posts a lot of similar messaging on both Twitter and Bluesky, rather than staying isolated to one or the other. It's not exactly the same thing, but it has a similar effect.

In an ideal world, everyone could easily host their own Mastodon server and just communicate with others without being tied to a platform, but unfortunately we still live in a world where the network effect is keeping people trapped in corporate social media silos, and there's only so much an individual politician can do to change that without harming their own ability to message to the public.