Just to be clear, the admin portal of WordPress core isn’t static. Calypso is external application from the actual core code that runs WordPress. Calypso is powered by the WordPress.com REST API while self hosted WP sites can also take advantage of this application via the Jetpack plugin. At this moment, WordPress itself has not transitioned (yet) to being ran on Node/React. But there is quite a bit of backbone powering the admin area.