There's a lot you can do with a custom reconciler. I didn't look to see if that's what's going on, but it's the foundation for many React adaptations to unlikely interfaces, such as Ink and some of Netflix's smart tv UIs.
It is a using custom renderer indeed: https://github.com/pmndrs/react-three-fiber it doesn't wrap Threejs, just expresses it via JSX similar to how react-dom expresses the dom.