Yep! It's a bit convoluted at the moment, and I should comment it better, but it basically just does the following:
1. Get a list of all posts and compute their URLs
2. Run each URL against the React app's react-router table (https://github.com/thomasboyt/peridot/blob/master/src/pagesB...). This gets the static markup for each route.
3. Wrap this static markup in a <Page /> container (which has e.g. <script> and <link> tags for assets), and write the resulting output to an HTML file.
That HTML file is what you initially load. It then loads a Webpack bundle of the same React app, and once that's loaded, React on the front-end "re-renders" into the already-rendered React app container (though it doesn't actually re-render the DOM, as it's able to diff against the existing DOM nodes).
There's some more nuance to this - for example, the server-side rendering also passes in a Redux store with the state needed for rendering the page (e.g. the contents of a post), as well as embedding the JSON-serialized state in a <script /> tag so when the front-end code takes over, it can just load the embedded state.
Honestly, though, the isomorphic rendering only took a few hours to implement, and most of that was me figuring out how to use Redux (previously, I'd used other Flux implementations that had somewhat different concepts). The biggest takeaway I've gotten from this project is that React isomorphic rendering is way easier than I thought.