Yes, I know the Italian league very well, I'm Italian!!! It's a tough and combative championship full of surprises, so Napoli can't yet consider themselves champions of Italy. Anything can happen especially after this World Cup. But it wouldn't be bad if Napoli won their third Scudetto this year.
Napoli indeed do not consider themselves to have won this season in Serie A because considering that there is still the second half of this season that has not yet started. And to keep Napoli's performance good or not after this year's world cup ends, it is also not certain because everything is still very possible to happen even though at this point Napoli have led the top of the standings with a fairly far points distance from their opponents.