Hi Tom -- When you add the layer to a map, the popup will be made based on the information (fields) in the map at the time. If you then add another field, it won't appear in the popup until you add it in for display, as you gave steps for in your post. In the blog, since you add all the fields before adding the layer to the map, you shouldn't need to go in and edit the popup. If you add the layer to the map earlier than the fields are added to the layer, then you will need to. I'm still seeing this behavior.
Esri Education Team