No. With ActivityPub, it's totally decentralized. There's no central authority whatsoever. What there is however, is an instance that's bigger than the rest. So if we talk about Lemmy. I can access it and post and my ISP can block Lemmy World and I'd never know.
With BlueSky, it pretends to be similar, but the reality is that everything needs to go through their central server in order to be displayed on a timeline. BlueSky is designed in such a way so that everyone does the heavy lifting, but to be seen, you need to have approval from their central server, where they can modify, insert adverts, do whatever. It's a centralised service that cosplays as decentralised. I don't understand why people keep pushing it when it's literally the same shit we all escaped from but with a new paint job.