Preseason are difficult to use as a benchmark for success but on the other hand it's also an early comparison when we're in the new season because it's like a situation where there will be more positive belief if a club does well in a friendly but on the other hand it can also be a worry and backfire when a club does badly in pre-season considering it's an early benchmark to judge how ready they are for the new season.
Indeed. We can't judge the performance of the team for the new season based on the performance of pre-season matches. In pre-season matches, most teams never play with their full strength because most players are second tier players. Pre-season matches are just for the time for trying alternative/new tactics and formation. We shouldn't be so surprised if there are some big teams that never get good results in pre-season matches because they probably don't target for the wins. Even if it looks like not a good sign but it is just usual thing in pre-season matches. The benchmark only can be visible from the results of few early matches in the league. Even if it is not 100% reflection of the whole season performance, but the early matches will be very crucial.
There are also some quite interesting things they are doing in the transfer market because AC Milan is able to recruit quality players and the depth of their squad is increasingly showing big changes from last season.
So far, I don't see big efforts of AC Milan in the current transfer windows. They only signed 3 players (Álex Jiménez, Álvaro Morata, Strahinja Pavlović). I only consider Morata as the most significant transfer until now. So, it is still debatable if you claim AC Milan sign quality players. If AC Milan has bigger number of players, it is because many loan players have returned.
Please check AC Milan current transfer: https://www.transfermarkt.com/ac-milan/transfers/verein/5