Besides it isn't even a correlation between math and short term memory. It is a correlation between math problems which require juggling a lot of data and short term memory. Note that the "juggling a lot of data" is a short term memory exercise.

Personally I believe that good short term memory is going to help in a lot of test situations. For a random example, on a verbal test you can read a passage and remember more of it as you answer questions.

Cheers,
Ben

PS Note that they didn't assume that there was a correlation between short-term memory and performance on the test. They measured that there was with their low-pressure test.