[vivekd]

Caffeine is an ubiquitous substance - so I wonder if these performance tests were done on people who had never used caffeine before, or at least use caffeine only rarely.

I'd assume the study would have told these people not to consume caffeine until they arrived at the study. If these people were regular caffeine users the positive effects could just be that the caffeine they were given alleviated the withdrawl symptoms of going without caffeine all day until the study.