It was not uncommon for Infantry to serve the guns if a Battery needed some help. In the
CS Army there were far more batteries of
Arty raised than there were guns to equip them, many artillerymen were trained as Infantry and thus utilized until the battery received guns. And Grants Overland Campaign put to use many Heavy
Arty units that had never seen a shot fired in anger.
It doesn't take long to train a man how to operate a gun... aiming and hitting something at range is a whole different matter. Armisted was likely thinking of Canister... any soldier can load a tin can w/ a powder bag into the muzzle of a 12 pounder and shove it home; at under fifty yards range aiming isn't needed to do a whole lot of execution among the enmy ranks.
Untrained canoneers will be loading and firing such a piece considerably slower than their better trained compatriots and if longer ranged firing was called for they would likely not have fared well when paired off against trained artillerymen.