Let me put it figuratively. You have taken the food - the FIRE of DIGESTION has flared up in the stomach. And after that you fill it with WATER.
Draw your own conclusions.
Today this statement is classified as a myth. If you do not drink a liter at a time, then nothing terrible will happen in the process of digesting food because of the water, the gastric juice will not dilute and you will digest the food calmly. Water, if you drink it before meals, during a feast or immediately after it, has little effect on the pH level (acidity) in your stomach, and drinking water at lunch or dinner practically does not affect the work of enzymes and the composition of gastric juice. This is supported by many experiments conducted by scientists and doctors with water and food. But if you generally consume little water per day, then you can get hold of digestive problems (constipation, etc.) because water, in fact, is the basis of saliva, gastric secretions, etc.