If we're going to look at this logically I suspect his getting to top the list has something to do with adding up inflation over some six-hundred years. It's probably equally why some of the secondary English nobles get to top the list over, say, Cornelius Vanderbilt. Because having one-hundred thousand dollars in 1406 and having one million dollars in 1806...well. I doubt they're equal but they're probably not as far off as you'd think.
Kind of makes me wonder what you'd get if you just tallied and looked at inflation for the last hundred years. Sadly I suspect most of the list would still be white and American.
That link up there will tell you a little about Mansa Musa I of Mali too, in case you're curious. I'm pretty sure he gets a mention in Crash Course World History too.
Happy Middle of Black History Month.