Is it true that the oil has the ability to stop moisture from entering the food and thus it acts as a barrier for moisture? 

Oil forms a layer over food products, creating a barrier between the food and the environment, including air and moisture. This isolation prevents the access of microbes to food and prevents them from growing and contaminating the food. The oil helps slow down the process of oxidation and evaporation, preserving the freshness and texture of the food. Additionally, the oil helps keep moisture from entering the food, preventing it from becoming dry or stale.