Sure, a salute in the surrender ceremony forced by Confederate military actions.
They were just as certainly casualties of war as were, for example, Hawaiian civilians killed during the Japanese attack by US projectiles fired at Japanese bombers.
In both cases, enemy actions were the root cause of those deaths.
You’re stretching. Even the NPS lists it as an accidental death and not a battle casualty.
“It’s interesting to note that Hough died an accidental death when a cannon discharged while he was loading it. This occurred the day after the battle ended, during a surrender ceremony”
https://www.nps.gov/fosu/faqs.htm