Thanks for sharing the photos. I am certainly no arms expert, but I would say that based on the description alone that they disqualified this as a Minié ball.
Rarest Miniball?
After extensive research this 100-caliber Civil War miniball is believed to be the only known surviving example test fired from a newly-developed REVOLVING CANNON at Arsenal Hill, Columbia South Carolina by General Wade Hampton near the end of the war
While it has the look of a Minié ball, it was fired from a revolving cannon and the Minié ball was muzzle loading weapon, not a Gatling style gun. For that reason I would say that it is similar in shape, but would have been self contained and therefore does not have all the aspects of a Minié ball.
Hopefully others can comment further.