Exactly Mitch!
I think the problem here is one of the conceptual problem people have of describing a solid.
3D CAD geometry does not have a way of defining the “solid” material between the inner and outer surfaces. It doesn’t matter whether you are milling or 3D printing. All that the output device cares about are the inner and outer surface definitions - the stuff in between can be considered “dark matter” 
If we imagine a model of a plastic drink cup lid, no one has a problem with this as it’s one fully enclosed surface and all the normals point outwards.
It’s when we have hollow objects such as a sphere with wall thickness where the inner and outer surfaces are separate and do not touch that the conceptual difficulty always seems to come up. All I can say is, “Don’t worry about it!”.
As long as the inner surface/mesh has normals pointing IN and the outer surface/mesh has the normals pointing OUT, it WILL print. Some software will ignore the inner surface though, so in those cases it’s best to add a small hole somewhere to join the whole thing into a single surface. Obviously you can’t mill such a thing as it stands but that’s a whole different story.
So, to reiterate: Don’t worry about the “dark matter” in between, all that matters are the inner and outer surface definitions.
hopefully that helps, Steve