Napoli did not make many major changes in its squad compared to other strong candidates so it is quite natural that they are still one of the strongest candidates for the scudetto and may be able to defend the title because if you look at Inter with Chivu for example, where Chivu cannot be said to be an experienced coach to manage a big team so I think he will have quite a hard time in Serie A later, maybe the most reliable to compete with Napoli is AC Milan.
I'm not sure you're missing something with Napoli, as they've been one of the most active teams in the transfer market so far. Napoli remains a strong contender for the Serie A title next season because they've been so enthusiastic about evaluating and bringing in a number of needed players. However I'm not so confident about Juventus or AC Milan's performance next season as a change in coach doesn't guarantee a significantly improved team.
Napoli and Inter Milan are still strong contenders for the Serie A title next season, and I'm confident that even though Inter Milan is coached by the inexperienced Chivu, they have the ability to compete for the title. I'm not so sure about AC Milan, but being able to compete in the top five of the standings until the end of the season is already much better.
Napoli, especially since they have been able to confirm Conte as manager, are still the best team together with Inter in my opinion. The real question is how Napoli are going to perform this year because they are going to play the Champions League: last year they were playing only one match per week, this year things will change, so they need more depth in the team, they need more players because Conte doesn't do a lot of rotation.
Juventus, Milan and Roma will simply fight for a Champions League spot.