Any team at a distance will lose to a top club for only one reason - squad depth/reserves. In almost every season, you can see a couple of bright teams that overtake the leaders but by the end of the season experience big failures and give way to leadership (exceptions like Leicester happen but very rarely). And this is only a matter of resources and not mentality. The mentality decides in knockout games.
Let's take Milan as an example, I don't think there is a very big, high-quality squad depth (like Inter or Juventus), but they had Ibrahimovic, this is a real leader with a winning mentality. Although he spent a lot of time on the bench, he is part of the team and he set the team up to win. Of course, it can be said that Milan had been knocked out of the European cups early and could have fully focused on Serie A, but even so, winning the Scudetto was not so easy.
Yes, but it may not be an easy task in all conditions. But if we compare seasons, this one was probably one of the most convenient for underdogs - Juventus spent half of the season in an undeniable crisis, while other leaders were unstable and changed at the top like in a kaleidoscope. Milan turned out to be the luckiest of all, and Ibrahimovic certainly has a partial merit in this.