this is described as "usually undefined" in QML in the documentation.

In the documentation page: http://doc.qt.io/qt-5/qtqml-javascript-hostenvironment.html

In the last section you provide an example in which you try to show that this is undefined in normal QML context.

However the example runs fine.

I think the sentence: "The value of this is currently undefined in QML in the majority of contexts." referes to a former current time.