Here, "covered", an active, transitive verb taking the direct object "ground", conveys vividly what the verbose and pale construction "were lying", coupled with the repositional phrase "on the ground", does not. Pullum's fussiness about the meaning of the word "passive" obscures S&W's point: vividness and brevity come together nicely in the active construction.
Pullum's point here is that neither phrase is passive construction, so you cannot point to the second one as better for that reason. The second sentence is better for the reasons you argued, but not because they eliminated the passive voice.
Yes, neither phrase is passive, in the technical grammatical sense, but the first is passive, as opposed to active: i.e. dull, inactive, indirect, circuitous.
Pullum's point here is that neither phrase is passive construction, so you cannot point to the second one as better for that reason. The second sentence is better for the reasons you argued, but not because they eliminated the passive voice.