Well, try not to get to technical when talking with these defintions. In reality, alternate picking is a form of economy picking. And that can get confusing when including tremolo picking in the scene. If you alternate picking though 3 note per string scales, you are already economy picking. Check out a lesson (not sure which one) by Schmange on his idea of making evey "box" type scale into 3 notes per string. That way, its up, down, up down... throught the entire scale. So the answer your question is YES. You do alternate pick, but when you do in that scense, its also economy picking.