power = work/time.

change some (fundamental physics)stuff around and you get

Power= torque*angular velocity.

Angular velocity varies directly with cadence, so
Power = torque*cadence to a constant.

This applies both at the cranks and at the hub on the wheel. The gearing system can trade out torque for cadence, but power will be the same under the came conditions (speed, incline, etc) regardless of cadence.