I am not sure the term is so strict and applies only to "controls" in GUIs.

Case in point: the Wikipedia page on skeuomorphism refers to objects outside of the domain of GUI language. It also covers physical objects referencing other physical objects (e.g.: skeuomorphic pottery, wood architecture imitating stone, plastic objects imitating metal, etc.)

Yes, but we're talking about digital interfaces right now.

Even when considering faux finishes on real world materials, that standard doesn't apply here. Unlike, say, the wooden shingles cut to look like stone on Colonial era architecture (e.g. George Washington's home at Mt. Vernon) that are trying to convince us they are something they aren't, the penguin icon is not trying to, nor would it ever, convince us it's a real penguin.

Going back to my first sentence, yes, skeuomorphism is a concept older than computer interfaces. When the term is applied to computer interfaces, it has to be adapted. Since current display tech could never create something even close to a convincing simulacrum of, say, a notebook, the term then gets adapted to mean that the use of skeuomorphism attempts to communicate functionally. Much like how "brutalist" Web design has nothing to do with the Brutalist architecture movement.