The meaning is different though. The one in the exercise could be a general statement. I would translate "They are fat olives" as "Illae (?) pingues olivae/olivae pingues sunt" in order to avoid creating too much confusion. It could be an alternative suggestion, sure, but I think it would be safer to leave it like this.
That's what I'm talking about, adding alternate translations. It wont replace the current translation, it will go from only saying "The olives are fat" to also include "Some olives are fat", "There are fat olives" etc. And the Latin side will include more options too.
The word lives on as «pingüe» in Spanish, «pingue» in Italian/Portuguese and «pinguid» in English! I had never heard pinguid as an adjective before, so a new word in bag, thanks to this Latin course.