I think the reasoning behind this is that react allows you to almost instantly see the result of code changes in your app. With c# you have to wait minutes for each build and deployment cycle, which is painful.
In terms of runtime performance, I guess in theory c# should be faster than javascript, but it depends on a lot of things, like the program being run, the speed of the mono runtime on the android or IOS platform being used, etc.
The reasoning is that with Xamarin you're still writing imperative code (AFAIK) whereas React is declarative. The result from a perspective of creating UI is that it's much faster and easier to create a UI in React. I don't think anyone would disagree with me here unless they haven't used React.
