No it isn't. They probably did something they are not telling us, like using skills in battle and not killing units manually.
Besides, the game had some spaghetti code before where there is damage reduction when you are above a certain level, which hurts overlevelling and benefits rushing. That's why KR players are able to clear stage 12-5 with just level 35 units by abusing this coding, which was then patched by the devs because too many KR players complained.
It's hard to be banned in story unless you used skills in battle.