Xamarin on iOS works in a similar way to Unity. They both use the Mono AOT compiler (ahead of time translation) to convert C# to native ARM code. It's not a JIT environment, like on Android. Miguel de Icaza is one of the founders of Xamarin and the Mono OSS project.

However, Mono isn't composed of ObjC classes so it's harder to detect just by looking at that list like the author did here. Xamarin does have a few ObjC components (such as XamarinNSThreadObject) since it adds a support library to the environment.

We actually match the structure of the code (control flow subgraphs), as well as other items like data references, so we get a more accurate view than you do just looking at class names.

You're right that each native or ObjC library you use with Xamarin has a plugin that exports a C# interface. It will be interesting to see how this evolves and if Xamarin will come up with a way of auto-generating these interfaces better.