But a number of things contribute to having the best defense, including ball management and possession. Milan doesn't have the best back four in Serie A but they had fewest goals against, so that should be rather telling.
I think Juve falling down, Inter not having a good first half of the season with Benitez, Napoli never becoming a contender were more essential in Milan's scudetto than anything mentioned here.
Look at Milan's CL results:
AC Milan 2 - 0 AJ Auxerre
Ajax 1 - 1 AC Milan
Real Madrid 2 - 0 AC Milan
AC Milan 2 - 2 Real Madrid
AJ Auxerre 0 - 2 AC Milan
AC Milan 0 - 2 Ajax
AC Milan 0 - 1 Tottenham Hotspur FC
In other words: they only won to Auxerre twice, drew RM due to Inzaghi once and drew Ajax due to an offside goal. Milan sucks bigtime even with their frontline, it's just this mediocre Italian league why they look good.