To be “more than an empty shell” means something is completely what it appears to be, or it has substance. Something that’s an empty shell is something hollow — it has only the external appearance of an object/person/idea but in truth there isn’t much too it. It’s similar to the expression that someone is a “shell of a their formal self,” which you might say if someone has been down in the dumps and not quite “themselves.” They look like themselves on the outside (their “shell”) but something intangible is missing inside.