"Do they have a good camera?"
Translation:Onko heillä hyvä kamera?
The partitive is used, when it is uncertain, whether the questionee has something at all. Usually in such cases you don't use any adjective attribute. Onko heillä kameraa? : Have they got any camera?
The nominative reveals, that the questioner already knows, that the questionee has some kind of ,in this, camera. Onko heillä hyvä kamera? : Have they got a good camera/Is their camera good?
Honestly it doesn't make much sense. If I ask "Do they have a good camera" (in any language), I don't know whether they have any camera at all, but I am only interested in good ones. They might not have any or they might have a bad one, in both cases I expect a "no" for an answer. If I do already know that they have one, and want to ask whether it's good, I'll ask just that -- "Is their camera good?". Even in oral communication, with the emphasis and everything, phrasing it as "Do they have a good camera?" is clumsy and easy to misinterpret, and in writing it doesn't work at all.
I hit the same deliberation and erred on the side of nominative here, but i think your point is valid, but the problem is most sentences where it is unambiguous (including your example "is their camera good") require the possessive which hasn't been covered at this point in the course. I think to give the team due credit they were trying to find an example sentence that works to demonstrate the nominative/partitive distinction discussed, but sticking with vocabulary we'd already encountered.
Partitive is not wrong here. Since there is no context, both nominative and partitive cases should be accepted, in my opinion. The difference in tone in this case is small, but already explained in this thread. Partitive is one of the trickiest things to learn in Finnish and without context it's often hard to know when to use it.