Given the ambiguity of this assumption, one that Luther seems to have made, a solid inference can be made about Luther's view of the supernatural. When in doubt, throw it out. Everything within the supernatural realm is ascribed either to God or the Devil. There are no third parties. Thus the only supernatural spaces are Heaven and Hell.
Supernatural agents are either angels or demons.
Ambiguity is at a minimum. This basic premise eventually fed or clarified a number of Protestant beliefs. The intercessionary powers of the Saints, and Mary in particular, were denied. Visions were suspect. All this was to come later, however.