I'm being a bit coy about it but the article aims to describe key ideas in the RSC wire protocol, which is an implementation detail of React and isn't actually

I'm being a bit coy about it but the article aims to describe key ideas in the RSC wire protocol, which is an implementation detail of React and isn't actually beholden to JSON itself. JSON is just a nice starting point to motivate it. However, I think reusing JSON for object notation kind of makes sense (and allows native JSON.parse calls for large objects).