[corsac]

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. It could easily be that e.g. the large gender difference between the gamer and non-gamer groups alone can account for the difference. Many likely confounders aren't mentioned in the study at 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).