Video games are more mentally stimulating than non-screen-based activities (building a fort, catching animals, reading a book)? People with better cognitive performance prefer more external mental stimulation? I didn't know any of that.

Your classic comment certainly stands, though. Based on how they recruited their testing sample, it could easily be that the gamer kids were just wealthier. Many likely confounders aren't mentioned in the study all.

But whether or not there's causation involved, this study tells us precious little about gaming and "cognitive performance" in general, since the stop-signal and n-back tasks they used have obvious connections to gaming but very little relevance to most other areas of cognitive functioning (of course kids who play video games for hours every day will probably respond quicker to which way an arrow on a computer screen is pointing).