The pickup is NOT a mic. Signal generation is by electrical induction in the coil caused by the string's vibration (read: motion) in the coil's magnetic field. "Sound" has NOTHING to do with how magnetic pickups work - they are completely 'deaf'. (At least the correctly potted ones are - but that's a different discussion
).
Consider that sound requires a medium through which to travel like air or water or rock, etc. In a vacuum, you could hit the strings on a guitar (acoustic or electric) and it would be dead silent. An electric guitar, though, WOULD produce a signal via the interaction of the moving string and the pups. Yes, even in a vacuum.
Read post #5 above...