17 lbs = 59,500 calories. 1 lb = 3500 calories.
That's 8500 per day, if it is true ... which I doubt for a couple reasons. Losing more than 6% of your body weight in a very short period of time (like a week) is dangerous. If you were anything less than 290 lbs to start with, you'd be well into that danger-zone. And most people don't put that much effort into losing weight.
Suppose that a person did eat enough calories per day to maintain their weight. In order to burn an additional 8500 calories per day, that person would have to cycle 17 hours per day ... or about a double century. That's doable for a few days in a row, but to keep it up day after day ... I don't know of many people who do that.
I have lost 8 lbs (which was about 6% of my body weight) in 3.5 days once, but I would not recommend it. I rode the RM1200 - 1200 kms in 88.5 hours. I was suffering some digestive difficulties during that time and could not eat solid food. I calculated that I burned about 44000 calories in those 3.5 days, but only consumed about 16,000 calories in that time. It took me two months to gain it back again.