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. 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.
Even though there is way more value in supporting Google, than Apple. Google SSO is widely used in small businesses, unlike Apple ID.