为什么在DotNet中创建XMLSerializer时，将接口类型成员解释为封闭接口？

Question

标题说明了一切，考虑到此程序：

using System;
using System.Xml.Serialization;

public interface ITest
{
    public sealed class Test //: ITest
    {
        public string TestString { get; set; }
    }
    
    string TestString { get; }
}

public sealed class Test : ITest
{
    public string TestString { get; set; }
}

public class Program
{
    public static void Main()
    {
        var serializer1 = new XmlSerializer(typeof(Test));
        Console.WriteLine($"{nameof(serializer1)} created.");
        var serializer2 = new XmlSerializer(typeof(ITest.Test));
        Console.WriteLine($"{nameof(serializer2)} created.");
    }
}

输出是：

serializer1 created.
Unhandled exception. System.NotSupportedException: Cannot serialize interface ITest.
   at System.Xml.Serialization.TypeDesc.CheckSupported()
   at System.Xml.Serialization.TypeScope.GetTypeDesc(Type type, MemberInfo source, Boolean directReference, Boolean throwOnError)
   at System.Xml.Serialization.TypeScope.ImportTypeDesc(Type type, MemberInfo memberInfo, Boolean directReference)
   at System.Xml.Serialization.TypeScope.GetTypeDesc(Type type, MemberInfo source, Boolean directReference, Boolean throwOnError)
   at System.Xml.Serialization.ModelScope.GetTypeModel(Type type, Boolean directReference)
   at System.Xml.Serialization.XmlReflectionImporter.ImportTypeMapping(Type type, XmlRootAttribute root, String defaultNamespace)
   at System.Xml.Serialization.XmlSerializer..ctor(Type type, String defaultNamespace)
   at System.Xml.Serialization.XmlSerializer..ctor(Type type)

但是itest.test不是接口，它是（混凝土）密封类！

这是怎么回事？可能是一个错误吗？