A boss rush is a mode in which you beat bosses back to back. This category is not that. It is using a password which you get from finishing the game (hence NG+) and then going through the levels to fight all the bosses (hence all bosses).

I agree that there could be a better name for this but NG+ has to be something strictly given by the game. if you have other suggestions go ahead :) (we have a Discord if you wanna discuss)