All the research I've done indicates that Milanos were originally Jews who lived in Spain. When the Spanish Inquisition forced them to flee, Milano Italy was one of the most popular places to resettle. Jews from Spain and Italy were absolutely notorious for simply taking the name of whatever place they settle in as their last name. So, in line with that, they just took Milano.
I know an individual who maintains one of the most popular and well known Jewish genealogy sites on the Internet. I was talking with him about it, and he said that he had an unpublished book of Sephardic (Jews from lands bordering the Mediterranean Sea, including Spain and Italy) Jewish names. When he looked up Milano, it was listed countless times throughout the book, leaving little doubt that it is definitely of Jewish origin.