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but even worse he failed to grasp the intensity of the individuals he faced. They were many, hardened, and extremely angry over the recent massacre of their civilians.

There were Indians from two groups which Custer would have expected to fight:
Free Indians, who had refused to go onto reservations, and who Custer was supposed to force to do so.
Reservation Indians who came out to fight alongside the others.

It isn't credible that Custer would have "failed to grasp the intensity" of those who would show up to fight, especially in light of attacks on Indian villages, killing of women and children and killing of their horses.

There were indeed "many, hardened, and extremely angry" warriors, among the world's best cavalry fighters, and Custer knew it. He just didn't know how many.