“Act as a decoy” vs. “act as _ decoy”: The Case of the Vanishing Article

In the particular instance of the Economist article quoted in the original post, I think Robusto's comment offers as good an answer as any. Indeed, the only other answer that occurs to me is that the author of the article was inconsistent by accident—a phenomenon that does occur in the real world.

However, I was intrigued by the possibility suggested by Edwin Ashworth's comment that something comparable may happen at the level of language at large—that is, English speakers may begin dropping the indefinite article from phrases whose underlying idea has become so familiar that the indefinite article may drop out of the phrase (at least on occasion).

To get an idea of when "act as decoy" may have emerged as an alternative to "act as a decoy," I ran an Ngram Viewer/Google Books search for the two phrases over the years 1750 to 1900. Though the resulting Ngram graph shows significant blips of occurrence soon after 1800 for both, the graph is quite deceptive in this case.

The first instance in the search results of "act as a decoy" in which decoy is a noun occurs in Robert Jackson, A Systematic View of the Formation, Discipline and Economy of Armies (1804):

The brilliancy of military dress, captivating the eye by its gaudiness and contrasts, entices the multitude to assume a military life: the dress is consequently ornamented and gilded that it may act as a decoy.

Google Books finds five other instances of "act as a decoy" for the period from 1804 through 1833—in four of which decoy functions as a noun—and many others through the end of the nineteenth century. In contrast, the first Google Books instance of "act as decoy" in which decoy functions as a noun doesn't occur until 1853, in John Francis, Annals, Anecdotes and Legends: A Chronicle of Life Assurance:

Though its board of management included some of the first bankers and merchants of the day, yet then, as now, it seemed necessary to catch a peer of the realm to act as decoy, so Lord Willoughby de Parham, with no interest in its movements or concern in its affairs, was paraded before the public as patron and director and at the end of two years was gravely thanked for the use of his name in maintaining the reputation of the novel society.

Four other instances of "act as decoy" with decoy as a noun occur in Google Books search between 1881 and 1888, and one more in 1899—and that's it for the nineteenth century. All of the other instances 0f "act as decoy" involve the use of decoy as an adjective, most frequently in the phrase "act as decoy ducks." Of course, none of those instances involve the dropping out of the indefinite article through ellipsis; they involve a phrase with a plural subject.

Still, "act as decoy," where decoy is a noun, does arise frequently enough from 1881 forward that the argument for ellipsis seems very plausible.

Something quite different happens in the case of "act as an envoy" versus "act as envoy": Again the Ngram chart for the period from 1750 to 1900 shows deceptively lively activity for both terms prior to 1860—but this time the false positives involve "act as an envoy."

The phrase "act as envoy" occurs as early as 1805, in Mercy Warren, History of the Rise, Progress and Termination of the American Revolution:, volume 6):

It was indeed doubted by many at the time, whether Mr. Dana was qualified to act as envoy at the court of Russia, and to negociate with such a potent state.

Numerous other examples of "act as envoy," from 1840 forward, also appear in the Google Books search results. Meanwhile, the first "act as an envoy" match in the Google Books results involves Walter Scott, A Legend of Montrose (1819):

Not forgetting that it was his object to collect information as well as to act as an envoy, and desirous, for his own sake, to ascertain Sir Duncan's reasons for sending him onward without his personal attendance, the Ritt-master enquired at the domestic, with all the precaution that his experience suggested, what were the reasons which detained Sir Duncan at home on the succeeding day.

For the remainder of the nineteenth century, Google Books finds only two non-Scott matches for "act as an envoy"—one from 1858 and one from 1874. This odd reversal of the "act as decoy"/"act as a decoy" pattern suggests that something other than ellipsis may be at work in particular instances of such pairs of phrases.