How interface provides decoupling to the client

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace Interface
{
  interface Interfface
  {

     void ADD();
     void SUB();

  }

  //Service Provider
  class ServiceProvider : Interfface
  {
    public void ADD()
    {
      Console.WriteLine(" I am ADD");
    }


    public void SUB()
    {
      Console.WriteLine(" I am Sub");
    }


  }


  class Program
  {
    // Client
    static void Main(string[] args)
    {

      Interfface Reff = new ServiceProvider();
      Reff.ADD();
      Reff.SUB();
     

      
    }
  }
}

Select allOpen in new window

Here the client is aware of Service Provider and Client he is creating reference to service provider.

There client is aware of service provider.
"   I have studied client need not to now the implementation part of service provider. "

But here client knows about service provider and client is also creating reference of it.

So I think client can get the logic of service provider by using Reflection concepts  in .NET.

Can you please help me  understanding this case.