I'm working on a site right now that doesn't do any SSR—it's just an index.html with a bundle.js on it. Google indexes it fine and surprisingly we have great SERPs. 2nd result for "trillion trees"

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.)