This study is strictly about whether you are more violent 10 years after playing the game.
If does not study at all the actual issue most of those who lived with excessive gamers (whether kid or adult) have. Meaning, annoying aggression right after playing and verbal abuse toward household members because they dare to exist if same flat where he lost the match.
Anecdotally, nearly all of my friends growing up played violent video games. I did see this behavior from one of my friends and his aggression following losing a video game. But he also had the same reaction after we lost a soccer game as well. I think that video games may show this effect, but it is not necessarily the cause. After all, all of my friends played the same violent video games (often together); the individual was the common factor in the aggressive activity, not the game.
In his case, his buddies in soccer either put up with it or don't. They have choice whether to go play with him. Also, soccer is played relatively rarely, like twice a week.
In case of gaming, people in the same householde are getting that despite not having much choice. It is also much more frequent - dedicated gamers play hours and hours every day.
Another aspect with soccer is that really only few people can do overnight, so again, the bad behavior due to sleep deprivation is less likely to happen. If course it can happen if you party while night or something.
These logistical differences can make big difference in terms of how annoying the hobby is for those who have to put up with people like your friend.
If does not study at all the actual issue most of those who lived with excessive gamers (whether kid or adult) have. Meaning, annoying aggression right after playing and verbal abuse toward household members because they dare to exist if same flat where he lost the match.