It is not certain but now Inter Milan are very much favored to win the Scudetto this season after they managed to beat Atalanta last weekend and that also proves that Inter Milan's quality is much better.
Regarding Napoli and Atalanta's undivided focus because both teams are not playing in the Champions League, I think it cannot be used as an excuse because Napoli's performance is quite bad now even though they are only focused on Serie A so it cannot be used as an excuse for big teams to share focus to win several titles.
Beating Atalanta at this period of season is very important result for Inter Milan and Atalanta seems to run out of energy and momentum in important weeks of season. I think Atalanta is more challenging with Inter Milan than Napoli because they have very powerful attacking system while Napoli have very weak attacking power. This weak attacking power has begun affecting Napoli more recent weeks when they failed to score one or two goals to win minimally like they did many times throughout this season.
Napoli success so far and their appearance in Scudetto competition is because of strong defense and ability of somehow winning minimally in many matches. It seems their momentum for it is running out too.