Read the NYTimes article... you learn that the two sets of pedals are 1:1 connected. Your partner pedals faster and you have to pedal faster. Maybe not harder - but you do have to keep pace.
Maybe we should design an exercise bike with motor, and you just put your feet in the pedals and let them roll around without applying any force yourself? Seriously, a fast cadence on a bike isn't even work if you're not applying any force yourself.
Alviva was wondering why the people on the back were not slacking. MediaBehavior says they were only pedaling faster, not necessarily harder. I'm pointing out that the article clearly says they were pedaling harder.