Back then, Otsutsuki Isshiki, who had been ambushed by Otsutsuki Kaguya and fed to the juvenile Ten-Tails, had not died completely.
Perhaps the juvenile Ten-Tails had found Otsutsuki Isshiki's hide too tough and coarse, far less tender than Kaguya's, or perhaps there had been some other reason.
In any case, after devouring half of Otsutsuki Isshiki, the juvenile Ten-Tails had stopped eating.
On the verge of death, he had ultimately shrunk his body and burrowed into a monk named Jigen. During the Boruto era, he had begun stirring up trouble again.
Since Yoru Machi had decided to go all out against the Otsutsuki Clan, he naturally had to nip such a hidden danger in the bud now that he remembered it.
So, empowered by Kaio-ken and using Aura Sense, Yoru Machi quickly spread his consciousness across the entire shinobi world, found the monk named Jigen, and brought death to him.
Otsutsuki Isshiki naturally reacted when he sensed Yoru Machi and Kaguya arriving.
Because he had been grievously wounded and left at death's door back then, he had been single-mindedly focused on surviving. Knowing that his kind were all equally cold and heartless, he had always intended to secretly plot for the Divine Tree's fruit while also seeking revenge against Kaguya.
But Isshiki himself had never expected that before his revenge could officially begin, Kaguya would come knocking first.
Sensing Otsutsuki Isshiki within Jigen's body, Kaguya was equally shocked. She had never imagined that Otsutsuki Isshiki had not died back then and had actually been living in her garden all this time.
Otsutsuki Isshiki had been her superior. He possessed the Byakugan and Black Eye dojutsu, along with many powerful divine techniques.
If she had not ambushed him back then, even several Kaguyas would not have been enough to defeat him.