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.
It would be very strange if Napoli thought of themselves as champions with many games remaining. They have to play consistently first to become champions. In terms of chances they are a club that has a very big chance to win the Scudetto this season. But they will not be able to freely and easily to make it happen, they still have a long way to go, some big clubs who are very likely to perform better in the second leg are a real threat to them. Napoli could have passed this half of the season very comfortably but that doesn't mean they will feel the same way in the second leg.