"They have yours."
Translation:De har deres.
The former uses the 2nd person singular version of the possessive, and the latter uses the 2nd person plural. Norwegian has different words for the singular and the plural, while English uses "yours" for either.
While you wouldn't make the distinction in written English, you can think of them as "yours" vs "you all's" if it helps.
I agree......they have theirs is not the same as they have yours without context you can't tell what it means. Per duolingo tips which say "Notice how deres can mean your (pl.) or their. You will be able to tell the difference through context." This is a time waster but it also shows that there are times that taking a raw sentence is just not translatable. Duolingo accepts both answers but does not present both answers or explains. This is just a tough call and why I have cut my time spent on Duolingo learning section and added google translate and rereading the tips and digging into the Duolingo dictionary for words an examples. It slows the learning process down but right now it's better than not learning at all.