For Napoli this season is not about winning the Serie A title anymore. They have realized by now that there are two teams that are out of reach for them. Juventus and Inter are not stoppable this season, at least not for Napoli.
But considering the other teams fighting for the Champions League qualification, Napoli is only three points away from the 4th position in the table. It looks really bad, but this is still absolutely possible for them. There will be a lot of interesting games for the 1st position in the table and then there is AC Milan, but after them there are so many teams that have a chance to claim 4th position.
Napoli can get started right away if they beat Lazio, but they are in good form with four consecutive wins.
Napoli's point distance is too far from first place, so Napoli prefers to play calmly and doesn't need to chase first place. They already won last season, so this season Napoli is giving other teams the opportunity to become champions in the Serie A League. Juventus and Inter are competing closely, and it looks like they are using all their strength to achieve victory, get additional points, and become champions in the league this season.
As if that is how teams go about their goals. A team that won the title last season will then say "who cares, we won last season, why should we try to win it again"... Yes, sure...
That is why Manchester City says they don't want to win the Champions League again because they won it last season already? Of course they did not say that, but it surprises me that you think this is how professional football or any sport for that matter works. The teams start to win the title and the teams start to defend the titles they won last season. As soon as a team would say that the goal isn't the title because they won it last season, it would disqualify them from the category of being a truly competitive and professional football team.
Napoli just realizes that at this point they won't have a chance, but that doesn't mean they are thinking for a second the way you just said they would think.