^^ this
Remember that madara has been on deaths doorstep before, so it's perfectly possible that he encountered the sage. This is why he knows so much; However, it's interesting that Madara wasn't told about the sage's brother.
I also believe Madara to be indra's last incarnation. Sasuke certainly isn't. he wasn't genius from birth, his brother was. Madara has been genius from birth. Madara was also blessed with powerful chakra that was renowned. Sasuke has developed his chakra to this point. Madara's parallels to indra are too close to deny.
well, saying "i also believe madara to be indra's last incarnation" in response to someone saying they think that he is Ashura's previous incarnation doesnt make sense.
But, what if Orochimaru is Indra's incarnation? He was stated to be a genius since birth, and has dedicated his life (and the lives of his vessels) to the knowledge of jutsu, so he has more power at his disposal...
But Madara being Ashura's incarnation before Naruto would be a huge twist that could happen. Unexpected because he is an Uchiha, but still possible.
OT: SO6P came way way way way before the current time. He probably met people we dont even know about. But he probably met with the sage animals and the incarnations of his sons through out time.