I'm guessing they are overloading the ref's usage with the custom renderer. I believe that a renderer can accept non-function/class components in theory, which is why this works.

I don't really like this though, although interesting, it feels like a hack and might not integrate well with tooling (e.g. the Typescript example).

How I would have implemented it was exposition a Mesh component which wraps this, and a useMesh hook to wrap and type the ref.