True! Maybe there is a little exception: you can use it with question words to question the question itself :)
-- Ne istiyorsun? -> What do you want?
++ Ne mi istiyorum? -> What do I want? (Are you really asking this?)
So "Arabayı kim mi kullanıyor?" would mean "Are you asking/wondering who is driving the car?"
That is a grammatically correct English sentence, but Duo in general wants you to use present simple tense to translate Turkish aorist, and present continuous to translate Turkish present continuous.
This one uses present continuous -ıyor and so it expects present continuous "is ...ing" in the translation.
In English there is a distinction between "who is using the car" and "who is driving the car". In the case of "who is using the car" you are asking "who is currently making use of the car". They may not be driving at the time when the question is asked. They may have gone on a trip and the car is parked somewhere. In the case of "who is driving the car" this indicates that the car is currently being driven and you want to know by whom.
Is there the same distinction in Turkish? Or kullanıyor only ever means "who is driving the car" in this context?