There are different kinds of soup in the world, leading to different ideas about how one consumes them. "Zuppa" is normally just a liquid or semi-liquid. One drinks a liquid. There are other types of "soups," though they would never be referenced by zuppa, in Italy which one would eat.
In English, the word "eat" has become synonymous with "consume." This, added to the diversity of soups, causes a great overlap of usage which can become erroneous. Few people will argue for the usage of "eat" in reference to plain broth, which illustrates the situation quite clearly.
I think the words "eat" or "drink" have to do with the way of consuming the soup.
"drink soup" means to lift the bowl to the lips and drink it like drinking water from a glass.
"eat soup" means you use a spoon to transport a little of the soup from the bowl which rests on the table to your mouth which stays up above the table and than "eat" it like you would eat something solid.
In China for example people lift the bowl to their lips and "drink soup". In Germany it is good manners to leave the bowl on the table and use a spoon to lift a spoon full of soup to the mouth.
How do you usually consume soup in Italy?
I haven't seen it explained here in Duolingo, but from my experience with other italian courses "Lei" can mean "You" when used in the formal speech. For example the informal way of saying "My name is Jorge and you?" is "Mi chiamo Jorge e tu?" now if you were to say that in a formal way it would be "Mi chiamo Jorge e Lei?" in such case "Lei" means you.
In my opinion even if it's on a plate and you are using a spoon it is still a liquid thus you drink it. That it may have other ingridients which you do eat? yes, but those ingridients aren't the soup they are just part of it. Think of it as a glass of lemonade with ice, you drink the lemonade and eat the ice.