Skeuomorphic is more than a tired cliche buzzword for pretend designers that want to voice their disagreement. It actually is a style for ornamental designs that mimic real-world things.
Three flat-colored circles in a row on a flat surface do not look like realistic items at all, they are actually the opposite, a stylized representation of a real life object.
in this context, it's being used more in the "spirit" than in the traditional direct physical resemblance.
In this context, the "spirit" is indicating that simulating a "mac window", which serves no major purpose in elucidating source code, which is analogous (but not strictly identical, of course) to simulating physical buttons in another context.
