I know the screen size so I can avoid image resizing
Why is this cool? It performs well and has native-like interactions (as in you can partially open-close the left nav). To do this you need to animate every frame with JS (no CSS transitions or animations here!)
Why doesn't it work without JS enabled? Because this is a JS tech demo. React does support server rendering so I could render to a static page with little to no code changes though.
The markup isn't semantic! The semantic-ness can be improved for sure. But we should care less about semantic-ness and more about performance or else the web will die at the hands of native mobile SDKs.
What's React got to do with it? React's one-way data binding is so performant that we can write the whole app declaratively. I don't think it's easy to do in other data binding frameworks.