If you check the API documentation for RhinoViewport and scroll down to the method ConstructionPlane() you’ll find that it indeed is an instance method - most are. Mousing over the icon also tells you that:
Static methods on a class overview are marked with a fat superscript s:
ActiveViewport gives you an instance of RhinoViewport. If you want a different viewport than you’ll have to get the parent view from the views table doc.Views from the example and then use the ActiveViewport property.