This is a more full-stack approach - Qt and Xamarin are geared more towards creating standalone apps. What you get here is the reactive back-end (plus web-site for free). This setup can be used for both data and UI - you could for example publish a new UI (not just content), and all native apps would automatically be updated on the fly.
If you take a look at the video - if you do this with Xamarin (or Qt) you would need to manually connect to a database or poll a RESTful service to see current data.
It's worth noting that if you wanted to create this kind of real-time app with a Xamarin frontend, you could do it quite easily with ASP.NET on the backend and SignalR.
That's certainly possible for the data part - but a lot harder for UI/code. With the method I'm using, you can actually push code AND UI to the client on the fly. As far as I know, that's a lot more difficult with Xamarin.
Also, the licensing is a lot more flexible than with Xamarin and Qt does provide support for more platforms.
It looks attractive, but at these times, the client application can be easily be auto-updated, that's how all applications work, including those made by Google.