You're right that four companies are going after SDK and Docs generation. After seeing enterprises like AWS, Stripe, Twilio, Palantir, Dropbox, and many more build codegen for REST APIs in-house, it seems like a no-brainer that they'll be a solution that serves the millions of developers out there working with REST APIs.
We're seeing a new category getting created before our eyes! I could see it being defined as "Developer Experience Infrastructure", "API Management", or "API Operations". Time will tell.
Edit: just took a look at the GitHub repo provided and turns out it's just a wrapper around their API.