When you consider that Hagoromo wasn't always JJ, then it all makes sense
Hago had Rinnegan (but not the 'rinnesharingan')
Hamura had the Tenseigan
This explains how they fought well side-by-side. If the ability difference was too high, they would have had trouble keeping sync. They were essentially equal until Hago became the Juubi Jin.
Humans simply never passed down the tale of Hamura because Hago intended to keep the Ootsutsuki Clan's presence on the moon a secret, and the legend itself was written such that Hago alone did it, hiding all traces of Hamura until Hago himself spilled it.
Just a theory based on canon. Not actually said anywhere.