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The forward digit test is a memory test. The backward digit test is a staple of IQ tests and correlates pretty well with the whole. I think the article used that because it was the best data available.

I came here to say exactly that. Since you already said it, I will merely add that it is bizarre that the article would veer into the IQ explanation after discussing self-selection, which is sufficient to explain the success of Indian Americans as well as the observed memory test scores.

112 was quoted as IQ score for Indian-Americans.

"When statistical adjustments are used to convert the backward digit span results to full-scale IQ scores, Indian Americans place at about 112"

I don't know how you can meaningfully relate a score on a "backward digit" memory test to an exact IQ score. Nevertheless, I am editing my comment to reflect that it's an IQ score derived from the test and not the score on the test.