You do not usually use target as a parameter because you would use the animate() method on ZIM objects. As in:
new Circle().center().animate();
Then the circle is the target.
But for using the ZIM DUO on with the animate function, you are right... usually it is obj as a param but that conflicts with the original obj that we replaced with props - but still keep obj around as legacy (that is what CreateJS use for their property object).