Originally Posted by
waterrockets
This is true only for measured power, if it's measured at the hub. Your actual power at the cleat doesn't change with gearing.
Look at the original statement. Speed remains constant; therefore, power to the road is constant. Since drive train losses change (slightly) with gearing, the only way for speed to stay constant is for power from the rider to change. This has nothing to with how power is measured or even that it be measured at all.