Nothing so scientific."Aggression" is probably defined somewhere in the study, and it was probably measured by "play this shooting game and then we're gonna observe you for the next X hours"
Whether or not the change in aggression is persistent over sustained periods of time is the issue. But it's not surprising. White adolescents raging over video games and sustaining that rage immediately post playtime is not surprising.
Examples of physical aggression included incidents such as hitting someone or being sent to the principal's office for fighting, and were based on self-reports by children, parents, teachers and peers.
Meta-analysis (the junk science of statistics), coupled with self-reporting means it's worth very little.