Is the singular form of "desiderata" a disused word?

I was interested in the following paragraph which appeared in an article titled “A New Gauge to See What’s Beyond Happiness" by John Tierney in The New York Times (May 16, 2011).

“They wanted to win for its own sake, even if it brought no positive emotion,” says Dr. Seligman, a professor of psychology at the University of Pennsylvania. “They were like hedge fund managers who just want to accumulate money and toys for their own sake. Watching them play, seeing them cheat, it kept hitting me that accomplishment is a human desiderata in itself.” [emphasize mine]

Can someone clarify if the fragment "a human desiderata" is "simply" ungrammatical, as I think it is, or if the problem consist in the fact that the singular form of "desiderata", that is desideratum, is a disused word?

Is it possible to argue the latter hypothesis from the nGram below?

Oxford Dictionaries - "desideratum: noun (plural desiderata)"

• 'a desiderata' is not formally correct in English (or 'unum desiderata' in Latin), because 'desideratum' is singular in English (as it is in Latin)
• borrowings from other languages aren't necessarily bound to the inflections of the source language (sometimes the rule goes with the source language, and other times with target, English)
• as to actual practice, I would think the better, more appropriate nGram comparison would be between 'a desiderata' and 'a desideratum':

which shows that 'a desideratum' is only waning in usage. 'a desiderata' does occur (by looking at the nGram for it by itself) but errors do occur.