Strong evidence? No. But the authors do not claim it as strong evidence. From near the end: Dr. Lieb said that there are two caveats to the results. One is tha

Strong evidence? No. But the authors do not claim it as strong evidence. From near the end: Dr. Lieb said that there are two caveats to the results. One is that they must be replicated by additional studies before it would be possible to say with some degree of certainty that the drugs enhance performance. The second was that the study contained a flaw: the games were too quick, creating the problem of time-forfeits in some games. Additional studies would need to have the subjects play longer games. The author's explanation is also not quite "their brains got better": The study’s conclusion addressed the additional thinking time as a critical component of the effect of the stimulants. The authors wrote, “This suggests that neuroenhancers do not enhance the quality of thinking and decision-making per time unit but improve the players’ ability or willingness to spend more time on a decision and hence to perform more thorough calculations.” Your explanation would, however, make an interesting control. If people can't effectively will themselves to spend more time, and further studies are able to repeat the effect when there is no time limit, then that would be stronger evidence.