get a lot of users by building good product, instead of restrictions

This is about locus of control: if twitter wants to do something about users hopping to and using a competitive platform, can they change some settings in twitter? Yes they can. Can they stop users activity on Mastodon or force them back? No, they can not. So they're doing what they can, rather than passively allowing it to continue.

They already have the users. It’s about stopping them from leaving. And Twitter is arguably a better product by miles than Mastodon.

Usability for people who don't use your product shouldn't really be a concern for someone building a product.