I've been thinking about this lately and I think I know the answer
It is because, until the sage was debuted and told his story, all we knew were legends and myths about him. Then the sage himself appeared and told the story of him and his brother, meaning there were two saviors in the fight to save humanity, all it was was an emphasis that there were two saviors so Sasuke could fit in as an equal savior to Naruto instead of just being a side kick. 2 modern saviors = naruto and sasuke.
Hagaromo filled in the holes in the legends, to Hagaromo, that story is completely normal.
