We see in this chapter that there were two children born to Kaguya. Both born with power and both fought and sealed the Juubi inside them. A legend is a story told that bears some facts but is usually altered as we see in this chapter. We always believed that the So6p was alone. Is it possible that his brother left the Uchiha tablet or better yet,his son ,Indra left the tablet and told everyone he was the So6p like Obito told everyone he was Madara. I believe Indra left the Uchiha tablet . After years of war with his own brother,his dojitsu probable advanced due to his hatred.