All true, but you're forgetting that Mono can AOT compile C# to native code using LLVM. That's how Xamarin.iOS works. So il2cpp doesn't really bring anything new to the table.
Once all code in a Unity project eventually ends up as C++ (even if it started as C#), they can leverage Emscripten, asm.js and related technologies to enable Unity devs to use C# in the editor, which is converted to C++, and ultimately Javascript. It seems crazy, but it might just work!
I'm not really sure that is their biggest issue; HTML5 publishing of Unity games seems mostly a novelty at this point.