Yep. The harder you ride, the more calories you burn, but the more calories that come from carbs. "Fat burning" is a low-intensity endeavor.

Averaging 278 watts in an hour uses 1 mJ of energy, which is roughly 1,000 calories an hour. Takes a lot of calories to keep that intensity up for very long. Suffice to say that significantly less wattage requires significantly less calories, so overeating can be a hard habit to break if weight loss is the goal. And though that should take place primarily off the bike, you don't want to be overeating on the bike, either.