No, I absolutely understand how federating sites work, and don't need the condescension.
It's common, in many types of federated reimplementations of non-federated sites, to create special-case bridges that allow treating a large non-federated site as if it were following the federation protocol, precisely to help people migrate and to continue interoperating with users who are on the non-federated site.
In the specific case of ActivityPub, there are multiple bridges that allow following Twitter users as if they were part of the fediverse. Multiple Mastodon users I know have mentioned them. People just don't talk about them very clearly, or document them in migration guides. Hence my comment suggesting that.
If specific portions of the fediverse want to avoid ever interacting with Twitter bridges, they can absolutely do that, just as they can avoid interacting with other portions of the fediverse. But it would be helpful if potential new users had some information they could use to migrate more smoothly.
That sounds a bit like "why would you even want to run Photoshop on Unix? It's not open source! It's an entirely different philosophy!"
Many people balance different needs and values. Someone might like the idea of open federation but still need to follow people on Twitter for their day job, or simply follow a thousand interesting people that they don't want to simply drop just like that, to enter into a comparative wasteland.
Providing people with half-way solutions actually encourages more people to come and transition smoothly.
The non-federated site has to play along with the federation standard for this to work. Just like Compuserve or AOL, which provided both proprietary "walled gardens" and (increasingly over time) interoperability with federated services like e-mail.
Because I am pretty sure it violates Twitter API use. Twitter has no incentive to allow data moving off their network.
Your question sounds like someone asking how a user of a newly opened Facebook account can follow people on Twitter. They're entirely different services. The federating, or not, is immaterial.
The first part is the alternative service.
Mastodon is an alternative to Twitter, presented in a Twitter-like format and flow. People who use it do so because they want to avoid the restrictions and cultures of Twitter. They don't want to post the same content across the two services. They don't want to interact with Twitter. They want to be on Mastodon.
The second part is the federating of the service.
Instances of Mastodon are spread around, hosting a set of users on their individual servers. Each of those instances can then communicate between with the others, intending for you to be able to follow and interact with users on other Mastodon server instances as transparently as those of your local Mastodon server. They are still interacting within the Mastodon environment.