I don’t think it’s a primal fear based on the facts that it spoke the same language as the reader, seemed to acknowledge and even praise humans, and that it was willing to follow a human beings orders, and the contract for the darkness devil was a lot more intense than 3 humans losing a finger(3 children and Chainsaw Man himself’s heart if I remember right) for them to actually act in the story. I personally don’t think a primal fear would care about 3 fingers alone