Good question. It made me think. It sounds more natural to say "nel piatto" rather than "sul piatto". If you say: "metti l'olio sul piatto", I'd hesitate a second and then I'd put the oil bottle on the plate, whereas with "metti l'olio nel piatto", I'd pour oil. Could it be that a plate is slightly concave and it's considered a sort of a bowl?
I think for this sentence both are interchangeable, but it may sound as "the cook has oil above(on top of) the plate" while nel is saying "in the plate" but like as part of the dish. I don't really know... Actually it's probably (nel) because (sul) hasn't been taught as a new word by Duolingo yet.