The article is referring only to employees that were hired and were also given the GMA, so qualification was not an issue.

However, using a coin flip rather than a GMA score on a prospective employee would imply a score of either 0 (missed 'em all) or 100 (perfect), and thus would likely have 0 correlation with the actual employee performance, rather than .51.

So your conclusion is right even if your analysis is not.