this is a common misconception with several things wrong with it:
1: "Spot reducing" or losing fat on a particular part of the body and not others, is impossible.
2: Bike riding DOES burn alot of calories, so it makes you lose fat on all parts of the body, but most people only notice it on their stomachs, thighs, or other "fat spots"
3: This claim is mainly made by severely overweight or older men, because it is extremely easy for men to lose fat around their middle. It is harder for most women because they have less fat reserves in their midsection and more on their hips, thighs, and chest; so this is where most of the fat is lost.
I would definitely reccomend bike riding because it is easy, efficient, and fun! Are you doing the "abs diet"? If so, they generally recommend this because it can help tone the abs (balancing) while helping to lose fat all over the body.