Becaue that binding is tigher than...

    (function(){ 
      var __that = this; 
      return function(){
        /*switch __thatthat/this ref*/
      }; 
    }());

The code in question isn't just a binding, and it will have other differences... The code above is similar to what babel does iirc.

Now, that said there are other differences, such as ensuring that let variables are properly scoped and const doesn't allow reassign... that doesn't even begin with the maps and other new strong types that require additional locking and checks that will be less performant in many scenarios.

It's more about stable software with understandable, manageable code more than it is absolute, raw performance.