[kitd]

Leg muscles are the most powerful in he body, but they are not used to the extent on a bike as they are on a rowing machine. Additionally, they extend the work done by the arms and back, ie the work is not just the sum of the individual muscle movements, but the combined action.

Yes, it is silly and the author intimates that in the OP and the article I linked too, but I don't think meaningful power generation is the point of the exercise.

BTW a decent average male should be able to do 100W easily for an hour. I'm a (not particularly powerful) rower in my 50s and can easily sustain 150W for an hour, ie circa 120W accounting for generator inefficiencies.