I'm not sure if the story Mendoza wrote is wrong but he was just following the source he got the story from. On page 125, describing a scene in the campaign for Knoxville, Mendoza wrote, "In one incident that day, a Federal shell landed in the center of the 5th South Carolina Regiment as the men advanced toward the enemy, tearing off the arm of Pvt. Robert McKnight. The severed arm flew through the air and struck the head of Pvt. Lorraine Swann, killing him instantly. Lt. J.D. McConnell remembered that portions of Swann's brain had splattered on his coat." Mendoza sites James J. Baldwin III's The Struck Eagle: A Biography of Brigadier General Micah Jenkins, and a History of the Fifth South Carolina Volunteers and the Palmetto Sharpshooters.
Baldwin wrote on page 248, "One bizarre incident occurred during the fighting when a Federal shell landed in the midst of Company E of the Fifth Regiment, tearing off the arm of Robert McKnight. The severed arm then flew through the air, striking Lorraine Swann in the head and killing him instantly." Baldwin actually reprints his source at the bottom of the page, I believe because it is not a readily available source. The citation reads, "McConnell, 'Recollections' 6. McConnell wrote: [A]t Campbell's Station, [the Federals] shelled us . . . and one shell killed four men and tore off Bob McKnight's arm. It struck Swann in the head and tore it to pieces scattering his brains on my coat."
I believe that when McConnell says "It struck" the "it" is the shell. He had been talking about the shelling, how it had just killed four people and tore off another guys arm. I think he was just further explaining the damage caused by the shell. I'm not sure its possible for an arm to be severed off with enough force to then shatter a guy's skull. The physics of the event seem impossible. Why Baldwin interpreted McConnell this way I do not know, perhaps the rest of the paragraph reveals why. And then I'm not sure why Mendoza accepted this story as fact without some critical thinking on it. He could have easily included in his footnote that this did not seem plausible but that he got the story from Baldwin.
Here is the page from Mendoza, the quote is about a third of the way down the page, at the end of the paragraph that is continuing from the previous page.
And here is the page from Baldwin. The quote is about halfway down the page and the citation is the next to last citation.