Got a source for MJ being the highest? What I'm seeing is The Beatles being the highest selling.Whatever he did, it was less the MJ and The Beatles.
MJ received the award for the biggest selling artist of the millennium, not Elvis or The Beatles.
What's more impressive is that MJ put out far less music than everyone else. He put out an album every 4-5 years, while other acts were releasing albums multiple times a year.
List of best-selling music artists - Wikipedia