"Soppa" can also mean something like "mess":
Vilken soppa. = What a mess.
Det var en riktig soppa. = It was a real mess.
...and in the above sentences you could replace "soppa" with "röra" (touch, move, stir, ..) for the same meaning. "Röra" is more flexible though: you could say "Det är rörigt" ("It's messy"), while "soppigt" is far less commonly used and would often sound weird. My home is certainly "rörigt" but not "soppigt" :)
We have checked this one with native English speaking English teachers and there should not be any problem with it. "A soup" would refer to a kind of soup or similar. As in "What is Minestrone? - It is a soup." It is of course not perfect, but for the sake of learning the article for soup, it is necessary.
In the example you gave, the article is used because you are describing a type of soup. The article goes with type (implied, even though it's not stated directly), not soup.
Some languages use articles where others don't, or where the use of the article implies something else. I've encountered some where saying "a soup" implies a bowl of soup instead.
So, here's my question. If you were in Sweden and wanted to say you were having soup for lunch, would you say en soppa? Or just soppa?
copied from another reply i did to a similar question: in some instances, "a soup" would be perfectly fine. For example, if your were ordering soup at a restaurant, you would most likely say "a soup" instead of "a bowl of soup". I definitely understand the confusion though.
There's an interesting discussion about this here: http://forum.wordreference.com/showthread.php?t=1230776
With uncountable nouns it is always possible to find a context in English where we can use the article "a", but seems like a bad idea for introductory language lessons. The fact that so many native speakers feel that it is wrong should be a good enough reason to drop it. There is no lack of foods for this lesson.
And if it's the principle of presenting genders, why didn't the lessons have "ett vatten" and "ett nötkött"?
I'm enjoying the lessons and think they're quite good. My remarks are only intended to help make them even better.
I especially value all of the extremely helpful discussions by native (and other) speakers. Thanks.