No misunderstanding, MEUG! I understand that egg whites have water in them, but as you add water to royal icing to adjust its consistency (which is a must for most techniques, unless you just add raw egg white), you dilute the concentration of albumen in the recipe regardless of whether you use raw egg white, powdered egg white or meringue powder! What matters wrt drying time and stability is the end ratio of water to albumen to sugar.
In any event, I see no material differences in the handling and drying of my royal icing when making these basic substitutions.