You're right. What I meant is, you don't need a third-party CMS provider. I suck at writing copy.
Some people have been using Outstatic without Vercel, and there's a section in the FAQs about it. But since this is the first version of the library and I haven't tested it with other services, I felt putting Vercel as a requirement would avoid people running into pitfalls with other services.
But oAuth is only setup with Github for now. So unfortunately that's pretty much a requirement.
I plan on adding support for other git providers in the future.