A gradient from one tone/tint/shade to another of the same hue implies one thing: directional light. If directional light is visible on an object, that object exists in 3D space, at least in the mind. If that object exists in 3D space, then it has tangible form. If a UI element has tangible form, instead of only function, then it is skeumorphic.

At least, that's what the article seems to be saying.