Opening in React devtools shows a number of hints[1]. "React Native Style Editor" section. "View" components with numeric "style" props. And the classname prefixes "css-" and "r-" match the generated identifiers from react-native-web[2].
This strong sign of the confidence Twitter has in React Native for Web[1]. In 2017, Nicolas Gallagher gave a talk[2] about why it was used for mobile.twitter.com.
[1]: https://imgur.com/lFUvL7P
[2]: https://github.com/necolas/react-native-web/blob/45f94eb43da...