I don't know if these are or aren't nerve controlled, I suspect it's going by the muscle movement you described
But let's assume they are in fact controlled by nerves
Most of the movement of your fingers actually comes from muscles in your forearm pulling on tendons that go into your fingers.
So assuming you wanted to hook a prosthetic up to the same nerves and such you'd have used for your real fingers, you'd still probably end up flexing your forearm muscles because that's where those nerves go