Cereal is a soup when defined culturally, since soup is generally defined as a liquid dish, and since cereal is almost always paired with milk, it has taken on the soup definition.
However, by strict definition, it is not a soup, but an ingredient of soup. Like Tomato Soup, Chicken Noodle Soup, etc.
Still, the cultural definition is more important, much like the word “driving.” It has in the past been used to describe controlling anything that moves, but has taken the exclusive “car” definition in recent decades(heck, cars didn’t exist when the word “driving” was invented). Cereal, by the same logic, has an overwhelming cultural context that defines it as a soup by pairing it with milk. To use the definitive definition, you have to say: “Cereal, by itself, with no milk.”
I welcome any challenges to this view, or add on to it.