React and similar JS frameworks were created by a walled-garden companies that do not want most their content indexed by Google. Use it on applications and content you don't want indexed with search engines.
It is perhaps possible that google indexed it from a previous version that did have html, or they are using a sitemap.
(I've just joined the project, and I intend to get them set up with proper SSR soon.)
It's that easy - just one liner to use it in ExpressJS and a server side template system.
Facebook indexes many sites themself for link preview functionality. Preview images, page title, description are all derived from open graph tags.