Everyone else heard Black Zetsu's cry before he was sealed, but their reactions differed. Katsuya Uchiha, for instance, had no idea who the mother Black Zetsu mentioned was. He was the agent Black Zetsu had put forward, but there were many things Black Zetsu had never told him—and could never tell him. As a result, Katsuya had enormous gaps in his knowledge of the situation. He did not even recognize the Totsuka Blade, believing that Black Zetsu had simply been destroyed by a beam of chakra.
Although he knew Black Zetsu possessed a degree of immortality, Katsuya also believed there was no way he could have survived Susanoo's attack. Had he recognized the Totsuka Blade, he might have thought differently, but he did not possess Black Zetsu's breadth of knowledge.
Kagami Uchiha saw things differently. Hearing the defiance in Black Zetsu's voice as he was sealed, he immediately became certain that this was indeed the one plotting against the reincarnates. The mother he mentioned instinctively reminded Kagami of Princess Kaguya from the myths.
According to legend, Princess Kaguya had been sealed by the Sage of Six Paths and his brother, and his brother had spent his entire life guarding the seal to prevent her return to the world. Records of this were extremely scarce. Kazemori Masaki had only found them after considerable effort and translated them for Kagami. It was only then that Kagami learned the Sage of Six Paths had a brother.
During their later discussions, Masaki had suggested that the mastermind's continual search for reincarnates and attempts to unite all-creation might serve more than the goal of recreating the Sage of Six Paths' power. Perhaps he also intended to break Princess Kaguya's seal.
Kagami had considered this conjecture too bold to put much faith in. One reason was