The point of the mythical man month is not that more people are necessarily worse for a project, it's just that adding them at the last minute doesn't work, because they take a while to get up to speed and existing project members are distracted while trying to help them.
It's true that a larger team, formed well in advance, is also less efficient per person, but they still can achieve more overall than small teams (sometimes).
It's true that a larger team, formed well in advance, is also less efficient per person, but they still can achieve more overall than small teams (sometimes).