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If you can give me another source where Madara and Obito could have learned about Indra and Ashura, we can continue from that, but there isn't any to my knowledge. Black Zetsu has the information, but since he was pretending to be Madara's will, he couldn't have told Madara or Obito anything from back then or else his disguise wouldn't be believable.
Either way, you have to understand that Hagoromo only addressed Ashura awakening chakra. This chakra is what gives him powers and what gives Indra his special eyes in the first place. Just the fact that Hagoromo says this and then says this chakra gave him power equivalent to Indra's is enough.