"The Unit" was aweful on multiple levels. But Mamet's point is basic and correct: Any scene, thus, which does not both advance the plot, and standalone (that is, dramatically, by itself, on its own merits) is either superfluous, or incorrectly written.
That line stuck out for me as well. I just finished the new Wheel of Time book the other day; It's much better than the last 5 or so for precisely this reason. An object lesson.
True for all literature really.