AC Milan played at home and also after the big changes this season, AC Milan's strength is much better compared to last season so I think it's not a big surprise for the victory that AC Milan managed to achieve over Napoli, it's very normal in my opinion.
This is a match between two top Serie A teams so a lot of things are different, and that was clear to see.
This surprised many people because of Milan's opening match result, having suffered a defeat against Cremonese. After that, Milan performed well. Honestly, I was also surprised that Milan managed to take a 2-goal lead in the first half against Napoli. Even after playing with 10 men on the field, they managed to maintain their lead despite so much pressure from the Napoli players. It was a victory from hard work, and I think Milan must have celebrated the win from that match.
It was a thoroughly deserved victory. Milan's players were given a huge morale boost by beating a team like Napoli with 10 men. There are teams that can't even compete with 11 players against Napoli. While Napoli dominated the match statistically, Milan defended well to secure the three points.