To repeat myself, I know that this is more than possible to implement. But you're also hiding a lot of complexity about redirecting from your app to a browser, managing state, custom url schemes, etc. If you want to turn your curl request into an actual app, there's some nontrivial code you have to write and test yourself. And this code is important - if a user can't sign in, your entire app is broken.

And to what benefit? This is the point of the article. Sign in for apple is extra work and extra complexity for no benefit (to the developer, at least). It's an immature project and the fact that Apple is putting in the bare minimum effort into the docs does not encourage me to adopt this feature.

Google, in comparison, has a working sample project and step by step guide for implementing Google sign in on iOS[1]. Google sign in is just as much a "curl request" as apple sign in, but they put in the effort to give a high quality, well integrated, and native example.

Apple can't be bothered, which discourages people like OP from adopting the feature.

[1] https://developers.google.com/identity/sign-in/ios/start