Uncooked rice is hygroscopic. It's not as hygroscopic as silica gel. An iPhone and most Android devices have some level of water resistance, which means they have sealed ports and sealed or glued cases and screens. Rice and silica gel will draw moisture out of devices that are not sealed. Even if a sealed device has a minor leak and moisture inside, it will not dry out for a long time. Ever had a watch with a crystal that fogs up when you sweat, go from the cold into a warm room, or after exposure to water? It takes forever to dry out if at all and the problem will come back.
What rice can do is draw moisture from the external parts of the electrical connector port, the mic, and the speakers. Maybe, if you open the SIM card and or SD cart port you can get some airflow going, depending on whether or not the card slots are truly open to the inside of the device. However, the problem will persist because you have a device that is not as water-resistant as you thought or because the seals have failed. Seals don't last more than a few years.
Apple is only interested in selling you a new phone so I would take their advice with a bag of rice grains. In any case, a phone that has gotten moisture inside is in my book no longer trustworthy. Once you get corrosion it tends to get worse. Get your data off it and replace it.