The details are classified, but just a look at the visible design and common sense about if the engineers were dumb will tell you they didn't only pay attention to a few frequencies. The coating isn't only (a variety of) split-ring resonators either, it sounds like there is a chemical component of some kind.

From what I've heard, any frequencies where they're weak will be very low ones, which both require bigger, more obvious equipment and are less precise. And even then it might be a matter of a basketball-sized signature rather than a golf ball-sized one, and will be somewhat directional.