I'm sure that Napoli lost this game on purpose. They are not in the best position in the Serie A standings and they need to keep playing in the Champions League. The Italian Cup is not important for them and they will not waste strength on it. The main thing for them is to improve their position in Serie A and try their luck in the Champions League.
Regardless of whether it was intentional or not, it was still quite disturbing to lose so badly even in a match that didn't really matter against a club that was weaker than them in every way, which was quite disappointing and a disgrace for a big club.
Indeed, for their reasons in this case it is quite understandable because the current situation they are still not very good in Serie A and prefer to look for better opportunities, namely the Champions League and Serie A rather than the Italian Cup but if this is deliberately losing with an embarrassing score like this I think something is not too wrong in the body of Napoli even though we know most of the players are young players who are given more opportunities to show their skills on the field.