Although the performance of Milan team is a bit erratic, I don't see any opportunity to underestimate this team at the moment. Milan team is strong. And Milan has proven it. I saw AC Milan's performance against Inter Milan in the Supercoppa. AC Milan won the match 3-0 despite being 2-0 behind. It is clear that AC Milan has the ability to defeat Inter Milan.
AC Milan are one of most successful clubs in Serie A and in Champions League, they are a most successful Italian club. They won Scudetto two seasons ago so it's no surprise that in their squad, there are enough quality players. Their performances can be bad because of their player bad forms but with their quality and experience, when they are united with each other and return to their nearly best form, AC Milan can return to be a beast in Serie A.
Inter Milan team is also strong. However, if they make any mistake in the match, they will lose points. In AC Milan's home match, AC Milan players will naturally be confident. There is no doubt that the match will be competitive. I probably won't bet on this match. Because it is difficult to predict the outcome of the match.
Inter Milan are stronger than AC Milan recent seasons and in this season as well. As you said, it is only one of factors, but in a direct match, Inter Milan can have big trouble with AC Milan. With their small difference in quality, performances of their players in a direct match will decide the winner which can be AC Milan or Inter Milan. AC Milan have chance to beat Inter Milan if they have good tactics and can wait patiently for opportunity during this big match.