Yes, I mean something other.
If the potatoes were more familiar than apples to the date of the sagas' codification, it could mean that the sagas (perfectly written by experienced writers, not by a known Icelandic farmer) were written after XV cent, in XVII or even later, up to XIX.
Like it sems to other ancient/medieval epics.
So, an "Idunn's golden potatoes" kenning would be a thing.
Just the poet didn't realize in time.