Many years ago as a kid, I remember reading a book called "The Sketchbook of the Revolutionary War Soldier" or something to that effect. I featured pen and ink drawings of uniforms, weapons, equipment, etc... Among the pictures were drawing of artifacts recovered at campsites that included bullets that had been modified to inflict additional harm. One had a cross cut in it to create a dum-dum effect. Another had an iron nail driven through it.

Assuming the description in the Richmond article is not a mistake, my guess is that some guy with an extra streak of malice was sitting around his camp bored and got the bright idea to create a "special" bullet to use the next time he went into action. The bullet described sounds like a ballistics nightmare, not something an arsenal would intentionally design.

Another possibility is that the bullet passed through something made of thin steel, such as a canteen or cup, and carried a piece of it into the wound. It happens with bits of clothing, so it would seem possible.

As for the accidental reloading theory below, I can't see someone pulling a round after a misfire and simply reloading it, especially if something broke off in it. In the heat of battle it would be much more likely that they would cuss upon breaking their ball puller, throw it and the messed up bullet on the ground, dump the faulty powder charge and load a whole new round.