So... tomato soup isn't soup either? :O
You make tomato soup from boiled tomatos and usually add some real vegetables or meatballs to it. Those salt powders you can add water too to make some synthetic soup isn't worthy of the definition of soup. Chemical waste is a better description.
But if you insist to just boil tomato's in water there isn't any vegetable in it. Although the same would go for mushroom soups so I think his definition of soup is off. Anyway soup is water based not milk based. So his answer was half right.
Please excuse my (probally) poor grammar