Communication rituals are not the same as summoning rituals.
Summoning rituals pinpoint a location, drawing down a divine being for the advent, where in the design of the ritual, both the altar and the person presiding over the ritual become coordinates for the divine being's descent.
However, communication rituals are just for the officiant to communicate with the divine being, so neither the ritual array nor the altar are designed to serve as fixed coordinates.
Since the God of Knowledge was summoned through a communication ritual by He Ao, although Elijah was unsure how He Ao managed it, the God of Knowledge's only coordinate for descent was now solely on He Ao himself.
To kill He Ao and cause the God of Knowledge to lose its point of traction for descent was the only way for the Grim Reaper to conserve enough power to continue descending.
