I think that your buddy with his buttons A and B is missing and what is also absent as counter argument is the total power available per BG-team.
Button A was the 5xFireball from one mage, Button B was 1xFireball from five mages.

Take a WSG, 10 people each. Make them mages. Team A has one guy with hack A. Team B has a multiboxer with five mages.
Team A has 14 simultaneous fireballs and the firepower of 14 mages.
Team B has 10 simultaneous fireballs and the firepower of 10 mages.

The hypothetical hack alters the power balance in between teams, the multiboxer does not.