They then accessed data for more than 5,000 of those children two years later. Over the intervening period, those in the study who reported spending more time than the norm on video games saw an increase of 2.5 IQ points above the average rise.
Correlation does not imply causation.
It's possible that higher IQ kids are naturally more drawn towards video games since video games let them solve puzzles and/or compete intellectually.
That said, I could believe a game such as Starcraft where fast thinking, hand eye coordination, and a healthy dose of game theory can improve cognitive abilities over a long period. After all, nature made it fun to play in order to practice our skillset, and video games are a form of playing. It's just that I've never seen a study that can conclusive prove this.