"They do not see the soup."
Translation:One nie widzą zupy.
Although your sentence makes some grammatical sense, it's very very weird. It would be: It is not them who sees soups.
"Nie one" is definitely "not them, but someone else". And than as 'widzieć' takes Accusative in positive sentences (and Genitive in negations, but it is not negated here, 'one' is), then zupy is the Accusative (and Nominative) for plural: soups.
The right case would be "którzy/które" (depends on their gender, first one's for "oni", the other's for "one"), but it doesn't really suit the sentence.
"To nie oni/one widzą zupę" nicely conveys "It is not them who sees the soup". So we don't really translate this 'who' part.