After reading their 'How It Works' page, it looks like they load the DOM into headless Chrome, let all the JavaScript events fire, wait for rendering to complet

After reading their 'How It Works' page, it looks like they load the DOM into headless Chrome, let all the JavaScript events fire, wait for rendering to complete, and then serialize the DOM afterwards. Whereas with just rendering the React component you're getting the resulting HTML but you're not seeing the effect of actually executing any referenced JavaScript and the resulting DOM how a browser would see it.