Seisovat means standing, not waiting anyhow. These translations are becoming quite derived, makes them difficult to get them passed,especially if they are translating words as other words!!
No, "Cars are waiting in a traffic jam" is the same as "Some cars are waiting in a traffic jam". More than one car, but not necessarily all.
The more idiomatic "Cars are stuck in traffic" gets 30,500 results from Google. "The cars are stuck in traffic" only gets 5,310 results. The use of indefinite cars, without "the", is not talking about all cars, all the time.
Consider these two sentences: "People are eating", and "People eat". The first refers to some indefinite amount of people, while the latter refers to all people everywhere. When an indefinite plural noun is the subject of a verb in the progressive aspect ("is" plus a verb with the -ing suffix), typically the subject isn't referring to all. Indefinite subjects of verbs in the simple aspect, like the second sentence, are more likely to have the "all" meaning.
As for Duolingo's sentence, "The cars are waiting in a traffic jam", a Google search brings up exactly two results: both from Duolingo.