The article explains that they rank the level of "self-made-ness" out of 10, with 10 being from a poor family with no advantages. Then it says " A total of 34 billionaires, or 8.5%, scored as 10s".

So given 10 possible rankings, it is pretty well distributed then?

Well distributed for what metric? Their backgrounds clearly don't match the normal population, since the normal population isn't 7% billionaires.