He could only act as a substitute in PvE.
There was more.
Back when Lin Ye was on his graduation trip, he interacted with the members of the Church of Healing.
He discovered that a Godkiller-level weapon like the hidden blade could not only devour the remnant soul of an Abyssal Demon, but it could also devour the soul of a summoned creature, permanently destroying a summoning card.
At that time, the woman from the Church of Healing summoned the Executioner to stop Lin Ye. However, after it was killed by the Cursemancer, a system notification suddenly appeared. It said that all its power had been absorbed by his blade and could no longer be summoned by the owner.
He just didn't know.
Was this a Godkiller ability unique to Hidden Moon, or could it be that all growth-type weapons were effective against summoned creatures? After killing them, he could take their power for himself.
In short.
The summoning card looked very powerful.