при таком коде возвращается true, false, false

На freecodecamp:

There is one crucial side effect of manually setting the prototype to a new object. It erases the constructor property! This property can be used to check which constructor function created the instance, but since the property has been overwritten, it now gives false results:

duck.constructor === Bird;
duck.constructor === Object;
duck instanceof Bird;

In order, these expressions would evaluate to false, true, and true.

https://www.freecodecamp.org/learn/javascript-algorithms-and-data-structures/object-oriented-programming/remember-to-set-the-constructor-property-when-changing-the-prototype

Подразумевался вышеидущий другой код? На самом уроке он не приводился, взял предыдущих уроков