For most people, visible abs do have to do with diet. But there are some with low bodyfat and no abs, because the ab muscles aren't sufficiently developed.
My abs used to be concave. No fat but no 6-pack either. Weak too. Doing weighted ab exercises helped bring them out and strengthen them, which helped all other aspects of my training.
Training abs with weights will make them grow just like any other muscle, so don't overdo it and get too thick in your midsection, but you need some development there to see abs and have functional strength.
You can also get an ab workout doing core strength exercises like squats and deadlifts, but this is like the chicken and the egg - you need ab strength to do them and you get ab strength from doing them.