They pushed the turret and level up together faster than the enemy jungler can come up with. The enemy jungler cant gank, even under turret, because Amumu is so strong early game AND has a level + ultimate advantage.
Together, these two players push the turret. That's all they do. They push the turret and farm minions together for 6 entire minutes straight. There's no counterplay - together they can burst down ANYONE without giving them any chance to respond or retaliate.
Amumu has effectively been playing top lane instead of jungle, leaving mid lane and bot lane without support whatsoever and giving up two drakes without resistance - just to lane with his duo partner in top lane and take turret plates.
They end up winning the game because the enemy top/jungle is so far behind and amumu is so far ahead they can't even dream of teamfighting. Amumu alone can now easily 1v2 or even 1v3 the enemy team because he is so far ahead in gold and exp from laning in top lane. There was never any counterplay or any chance to make a comeback.
Is this reportable? Amumu has basically been playing top lane instead of jungle, without the approval of his team. Sure, he did carry the game - but he sacrificed ALL objectives and both the other lanes for his duoQ strategy.
This seems sort of like the smite singed/nunu "support" thing a couple years back.