I keep getting amazed by the opinions of people on this.
Some say its plothole , he couldn't have read it because he was linked to the Mazo.
Other say he did it via Zetsu.
But no one say that he could have read it himself with the Rinnegan , how you say ?
Well when he awakened the Rinnegan , he did break the Juubi seal and bring his body ( the GM) l to the earth and then connected himself to it , and thus in that period of time he could live without the need to be linked to the GM.
So we can conclude that the moment he awakened the Rinnegan , he did go and read the Uchiha tablet and THEN proceed to unseal the Juubi.