I've heard two explanations. 1. Piatto can also be what we'd call a "dish" in Britain - a deep plate, e.g. for pasta - hence "in" for deep and "su" for flat. 2. "su" implies sitting or lying on - "upon" in English - and hence is correct when the plate is upside down (!) whereas "in" is not.
I like the "upon" vs. "on" distinction. Could an Italian native comment please?
[Edit] The guardians of the language (Accademia della Crusca) mention disporre, mettere, adagiare, posizionare nel/sul piatto, implying they are interchangeable when talking about physical placement. Duo shouldn't be rejecting either in that context. However, piatto also means "dish" in the culinary sense, in which context only nel and "in" are correct, and "plate" is wrong unless qualified (e.g. "a plate of chips").
This is the second time in Level 4 of Prepositions that I have seen 'sul piatto' whereas in all previous sections including Food and Food 2 it has always, always been 'nel piatto'. Comments from native Italians also say it should be 'nel piatto' so I will report it but I would similarly love to know why it has changed here. Maybe there is something I am missing.