Why does Microsoft advise against readonly fields with mutable values?

Question

In the Design Guidelines for Developing Class Libraries, Microsoft say:

Do not assign instances of mutable types to read-only fields.

Answer 1

This is legal in C# (simple Console App)

readonly static object[] x = new object[2] { true, false };
    static void Main(string[] args)
    {
        Console.WriteLine("Hello World!");
        x[0] = false;
        x[1] = true;

        Console.WriteLine("{0} {1}", x[0], x[1]); //prints "false true"
        Console.ReadLine();
    }

that would work. but that wouldn't make sense. bear in mind the variable x is readonly, and has not changed (i.e. the ref of x has not changed indeed). but that's not what we meant when we said "readonly x", is it? so don't use readonly fields with mutable values. It's confusing and counter-intuitive.