I think they didn't thoroughly think about the effect the heat changes would have on damage. In their mind they probably just thought they are increasing MCH dmg by 3%, allowing for more overheat opportunities, and making heat management easier. All positives right? They didn't put all the pieces of the puzzle they were changing together and say how it effects the overall rotation/dmg. I can only assume the person who made the balance changes isn't an actual MCH player.