Electric superchargers are generally lumped in with Tornado "vortex generators", clamping magnets to your fuel lines, and other "miracle" performance products.
This one is probably a little worse though, since it draws quite a bit of power. Offhand I don't remember what size alternator is in the Nitro, but I'd be surprised if it had 50+ amps worth of excess capacity. The oversize heavy duty alternator in my V10 Ram was "only" 196 amps. I would expect the Nitro to be down around 100 amps.
Additionally, you have to consider that a vehicle like the Nitro has very significant driveline power losses. If they're claiming a MAXIMUM 15 flywheel horsepower gain, you're talking about MAYBE an additional 1 HP at the wheels. For the average dyno, that's essentially unmeasurable. You'll see bigger horsepower fluctuations than that due to minor temperature and humidity changes throughout the day.
Finally, the most obvious point to consider is that if it was really this easy to make a little extra power, you'd have seen it on a production vehicle already. After all, manufacturers do regularly sell turbocharged and supercharged vehicles, and it isn't uncommon for "better" models to have only modest power gains in the 15 or 20 HP range.