That's because it is not a mirror episode. It is an alternate reality/timeline episode, much like Parallels. Mirror episodes character bend. Hence the reference to it being like a mirror... backwards. So you have "Evil" or audacious versions of characters etc... In Yesterday's Enterprise, one altered event in history changed everyone's lives... It didn't change everyone. They are all still the same people...Thank god.Honestly, there's only ever been one Mirror episode that's ever been worth it; Yesterday's Enterprise.