I think this is the part you're not getting. The set of people who use Twitter is not a strict subset of the set of people who use Facebook. There are people who use Twitter and not Facebook or vice versa. A large number of people.
> This is a bit of a catch-22, no? Your 'entire audience' is everyone that you can reach.
Not at all. Your 'entire audience' is everybody everywhere. If a given platform has a monopoly over any subset of people, they have a monopoly. Some people use Twitter and Facebook and Reddit, and none of them have a monopoly over that subset of people, but some people use only Twitter. And if Twitter was federated (like email) this wouldn't happen at all, because there would be no users exclusive to Twitter since you could reach all the same users using any other provider of the federated protocol.
> If Twitter objects to your content, they can bar you from their (relatively small, to point out the analogy again) share of 'sidewalks', but there are still a vast number of other sidewalks 'in town' that you can advertise on.
Which, again, the same people don't walk down, so you can no longer speak to them.
> You are perfectly capable of reaching anyone who uses Twitter through another social media service, or setting up your own website/newsletter/Discord channel/podcast/blog/etc.
You're saying that the company town doesn't deny you from speaking to anyone on the sidewalks of another town, so you can just have the people you want to speak to go to another town and speak to them there. The obvious problem in this case being that the network effect prevents people from doing this. You would have to convince everyone to move at once or hardly anyone does.