I'm always a bit amazed by claims like this. I'll start by saying that it is POSSIBLE to burn 800 calories per hour while cycling. However, it is unlikely that a person without a high level of cardiovascular fitness can achieve an energy expenditure in that range, Here's how the math breaks down:
In order to burn 800 calories in an hour, a person needs to burn 800/60 or 13.34 Calories per minute. Since humans use ~5 Calories for every 1 liter of oxygen consumed that means that a person would have to AVERAGE at least 2.67 liters of oxygen consumption per minute to burn 800 calories in an hour.
-The equation if one was inclined to do the math is: (((10.8 x Watts) / Body Weight in Kg + 7) x Body Weight)/1000 = Liters per minute. So, a 70 KG person (154 lbs) pedaling at a work rate of 200 watts would be using about 2.65 liters/ min. or roughly 800 Calories. A heavier person would presumably require less energy, and a lighter person would require more.
To put this in context, most riders in the tour de france average slightly more than 200 watts on any given stage. In fact, most of the professional females I have worked with are not able to sustain workloads greater than 200 watts for more than a few minutes.
So, while it is not impossible for a STRONG rider to burn 800 calories for an hour, most would be doing well to reach the 500-600 range for an hour long ride.
Now, if they try to tell you it's because of high intensity interval training or some such thing, the bottom line still applies: 1 Liter of O2 for every 5 Calories.
I hope that my long winded response was helpful rather than confusing!