You shouldn't dismiss this off hand. It is possible to build such a device.
Just imagine for a minute being able to charge the battery on the down hill stages and then using that "charge" on the flats and climbs.
Pro cyclists have always looked for an edge... Performance enhancing drugs have been the norm for decades.
But I suspect it would be very easy to find this device in a tech inspection.
Meanwhile I did some searching and came up with this.
--- The fact that the officials are aware of the allegations and doing something about it tells you there could be some truth to this.
"Elaborating on the UCI's preventative measures, Marco Bognetti, consultant to Jean Wauthier, the current head of the body's materials unit, said:
"We've discovered that it [the motor] could save a rider between 60 and 100 watts, which is an enormous advantage in the finale of a race.
Our technicians are working on a special scanner that will discover hidden motors inside the frames. All the bikes at the major races will soon be checked."
Cycling officials dismiss claims of 'mechanical doping' in the sport | Sport | guardian.co.uk