I very much agree with you. Because the difference in rankings in Serie A and UCL alone can explain the current quality of both. Atalanta is currently ranked 2nd in Serie A and in the UCL Atalanta is ranked 5th. Yes, that is a very good ranking which makes it clear to us that Atalanta's quality this season is truly extraordinary.
Meanwhile, AC Milan is still struggling to recover with their new coach. And even though AC Milan has improved recently and they even made three consecutive wins in the UCL recently, including beating Real Madrid, their consistency is still in doubt. AC Milan in the UCL is currently ranked 16th. Meanwhile in Serie A AC Milan is ranked 7th. Well that's not a good ranking at all. And favoring Atalanta as hosts in this match seems like I would prefer.
And after 14 rounds, Atalanta is ahead of Milan by 10 points, which is also a lot. The teams are currently solving different problems, Atalanta is obviously trying to fight with Napoli for first place, and Milan is 6 points away from the top 4. And this will also be a home match for Atalanta, which will also give them an advantage. Milan cannot boast of great stability, by the way, this has always been a long-standing problem for Atalanta, but it seems that Gasparini is doing a better job this season, at least for now.