How do Naruto and Sasuke win "extreme high-difficulty", again? They won against a much stronger being that had an entirely new set of attacks, and those were 50% Kurama Naruto and inexperienced Rinnegan Sasuke. Hagoromo's just got the basic Rinnegan set of attacks, which N&S have already dealt with, and maybe a unique to him technique, which won't be enough to beat both of them together. Current Naruto and Sasuke mid diff him.
Umm, no? Naruto alone isn't on Hagoromo's level, not even close. How does a 9-tailed beast host with no eyes equal the 10-tailed beast host with dual Rinnegans? It's the same with Sasuke, he might have a special Rinnegan, but he's got no Biju. Each of them lacks something to beat him individually.
It's fairly obvious that Naruto took his individual power far past the standard limits. No juubi jin has demonstrated an avatar of chakra. No juubi jin has absorbed so much natural energy, that they almost overload. Half Kurama Naruto matched the biju susanoo, which is pretty much all of Hagoromo's yin chakra through susanoo merged with the biju's chakra perfectly. Needless to say, Sasuke was in the same ballpark as Hagoromo in brute power. Naruto matched that with half Kurama, and then got full Kurama, which obvious gives him a boost as Hagoromo wanted yang Kurama to defend Naruto. Naruto's, at the very least, on Hagoromo's basic level.