text/koot_nightwatch/3 matches the site

public static LinguisticObject Koot_About()
{
    var koot = new LinguisticObject()
        .WithContext()
        .WithId("https://linked.art/example/text/koot_nightwatch/3")
        .WithLabel("Content of Koot's Night Watch")
        .WithClassifiedAs(Aat.Monograph, Aat.TypeOfWork);

    koot.About = [
        new HumanMadeObject()
            .WithId("https://linked.art/example/object/nightwatch")
            .WithLabel("The Night Watch")
    ];

    return koot;
}

• Metatype classification The second argument classifies the first, not the object: WithClassifiedAs(Aat.Description, Aat.Statement) says this is a Description, and Description is a kind of Statement. The Plain C# form shows the nesting.

var linguisticObject = new LinguisticObject
{
    Context = ["https://linked.art/ns/v1/linked-art.json"],
    Id = "https://linked.art/example/text/koot_nightwatch/3",
    Label = "Content of Koot's Night Watch",
    ClassifiedAs =
    [
        new Type("http://vocab.getty.edu/aat/300060417", "Monograph")
        {
            ClassifiedAs = [Aat.TypeOfWork],
        },
    ],
    About =
    [
        new HumanMadeObject
        {
            Id = "https://linked.art/example/object/nightwatch",
            Label = "The Night Watch",
        },
    ],
};

Generated from the object the example builds: every property that is set, as object initialisers, with the vocabulary constants where a term is in the registry. The fluent helpers are a convenience, not a requirement; this form and the one as written produce the same JSON.