They’re right. Where your fat is isn’t based on areas that don’t get exercise, it’s just based on genetics. Some people carry most of their fat in their butt or hips, some in their belly, some basically everywhere. You’ll lose fat from wherever it is if you burn more calories than you take in (with some weirdness as your body works for a while to retain it’s current amount of stored fat).
So any exercise you get will contribute to burning more. You could get one of those exercise things that’s basically bike pedals you work with your arms. Then the other side of the equation is to take in fewer calories.
One other note: you can have little body fat and still not have a flat stomach. If you have no tone in your stomach muscles, your tummy can look like you have a bit of a belly, even if it’s not fat. Even just going around with your tummy held flat can help tone up those muscles if you can do that.