I disagree. That's taking the definition of Skeuomorphs and stretching it as thin as you can.
The purest definition of a skeuomorph is that of something which projects a ornament or element in order to imitate the actual existence of the object". For a non-computing example, my mom's car has Mercedes "MB-Tex". It looks like leather, it feels like leather, it acts like leather, but it's not leather.
As far as the case of buttons on OS X goes, I would argue they aren't skeuomorphic at all. They're flat with little to no roundness, and they aren't really "emulating" any button or switch in real life. If you wanted to argue that they were trying to emulate real life, then they'd look more like this:
To say shadows are skeuomorphic is kind of stretching it thin too. "Windows" don't cast shadows in real life.