My thinking is that you ride your bike because you like to ride. The side benefit of riding is that it will slightly increase your metabolism. This increased metabolism along with limiting the calories eaten will help you lose weight. You could probably get by with riding 30 minutes 3 times a week and be fine with the exercise requirement if these 30 minutes were pretty hard riding which got the heart rate up to the 80 % mark.
I need to qualify what I have said by saying that I really don't know what I am talking about.
Good luck with your weight loss. As you make your journey remember that if you get off track all you have to do is just get right back on program and continue. Most will be side tracked from time to time so don't beat yourself up if this happens.