I've never sweetened chicken soup either. (I will sometimes even use a little vinegar, though that's partly for health.) If I want sweetness in a soup, I'll add onion or carrot. But cooked celery is so bitter! My sense is that the sweetener is just to alleviate the bitterness of the celery. Back when I did dairy, my concept of cream of celery soup involved about as much onion and leek as celery.