Непонятный код из примера с сайта MSDN. Реализация IEnumerable

IEnumerator IEnumerable.GetEnumerator()
{
    return (IEnumerator) GetEnumerator();
}
 
public PeopleEnum GetEnumerator()
{
    return new PeopleEnum(_peolpe);
}

public IEnumerator GetEnumerator()
{
    return new PeopleEnum(_people);
}

public interface IEnumerable<out T> : IEnumerable
  {
    /// <summary>Returns an enumerator that iterates through the collection.</summary>
    /// <returns>An enumerator that can be used to iterate through the collection.</returns>
    new IEnumerator<T> GetEnumerator();
  }

People peopleList = new People(peopleArray);
IEnumerable ienumerable = (IEnumerable) peopleList;
     foreach (Person p in ienumerable)
        Console.WriteLine(p.firstName + " " + p.lastName);

IEnumerable people = peopleList;
foreach (var person in people)
{
   Console.WriteLine(p.firstName + " " + p.lastName); // Ошибка компиляции: у object нет свойств firstName и lastName, нужно кастить.
}

Person[] peopleArray = new Person[3]
        {
            new Person("John", "Smith"),
            new Person("Jim", "Johnson"),
            new Person("Sue", "Rabon"),
        };
 
        People peopleList = new People(peopleArray);
         // используется публичный GetEnumerator, не?
        foreach (var p in peopleList)
            Console.WriteLine(p.firstName + " " + p.lastName);