Does it actually do static type checking? Because I don't see anything about types in the documentation. It looks like just a more sophisticated linter, which is very different from what TypeScript and Flow bring to the table.

Ah, you're right. It's "just" a linter... but a pretty good one.

I started using rubocop earlier this year and I've become a huge fan. It's done a lot to improve the quality of my code and the consistency of code on projects with multiple developers.