This tagging is not made to be used for each single building, but for the area on which the buildings are standing (here the buildings are used for apartments).
You couldn’t see anything new on the map after adding landuse=residential here for the first time, because there is a larger landuse=residential (relation https://www.openstreetmap.org/relation/9110471 ) in this town, which is drawn with the same colour as your input on the OSM Standard Map.

I think iD is right, but there can be a misunderstanding if you see: a) the area on which there are apartments or b) the apartment buildings themself. Here the iD developpers thought at case a).