Someone with a knife at your back has the advantage. If you try to pull a gun, they still have the advantage. It's not impossible for you to kill them, but it's unlikely. I use the 21 foot rule not as some sort of magical barrier within which you are automatically dead, but rather it's a way to demonstrate that even a trained shooter is in serious trouble if their gun isn't out and ready before things become close-quarters. A civilian probably worse.