Your parent is right: IQ has been increasing. Even in countries where you wouldn't expect the cause to be improved nutrition. But nobody really knows why, nor has anybody discovered an intervention that can, on its own, reliably increase IQ.

Not only nutrition, but overall childhood experience is getting better worldwide. Even in countries that had no malnutrition, there was a lot of space to improve in children<->parents relationships etc.

Meanwhile some countries that were really good in all areas a while ago are now reporting minor IQ decrease..