As history becomes legend and legend becomes myths.. We've seen first hand from the sage that this has happened before. The genjutsu was cast as well as the tree sharing everyone's life force. The only two who were unaffected by Kaguya were the sage and his brother. So why didn't the sage just say how they defeated this genjutsu ? Yes plot I guess. But you'd think he'd mention the fact they fought against it and won but more importantly how ? As the plot progresses it only makes the sage look more and more idiotic, his choices, foresight, and non preparation for all of this have all been negative..
