grammatical number - 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?

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