Rob, that was a good example why one shouldn't trust everything they read on the Internet, and especially Wikipedia.
That was not the definition of negative space in photography, and not what I had in mind. What I did have in mind is much simpler: when confronted with a "featureless" sky, ie, no spectacular-looking clouds, one might want to experiment and include more, not less of it. In the OP case, such a sky at least had nice colors. By turning the camera upright (in absence of a wider lens), and placing the horizon in the lower third or even fifth of the composition, one would get a nicely colored, "empty" sky (thus "negative" space), which would, paradoxically, focus the attention to the lower parts of the image. If you want photographic simplicity, you can hardly beat that.