How did Madara read the Uchiha's Stone Tablet about the history?
It's not a stretch to think that he managed to sneak back inside the village and read it in his spare time.

Somewhere before he connected himself to the Gedo Mazo and after he awakened the Rinnegan.

It is not explained however, so any answer you'll get here would be speculative.

EDIT: Now I understood your question, however, nothing told you that the part about the God Tree is only readable by the Rinnegan, maybe it was a part of the tablet that could be read using a Mangekyo.