Indeed, Inter Milan managed to move away from Napoli or lead by 3 points but don't forget Inter Milan have some important matches ahead such as the Champions League and the Copa so this will have an impact on their performance in Serie A well, this is an opportunity for Napoli to catch up or take over the top of the standings. The Serie A title race is still open for both teams and we can't confirm yet which team is truly safe, while Atalanta could be excluded from the race imo because their efforts do not come close to those of their competitors.
You make good point that Inter Milan has some tough games coming up in Champions League and Coppa Italia which could affect how well they do in Serie A. This gives Napoli chance to catch up or even move into first place. Competition for Serie A title is still very close between Inter Milan and Napol and it is too soon to say which team is in lead. Atalanta on the other hand seems to be falling behind because they are not playing as well as Inter Milan and Napoli.