FYI, I solved this with the help of a friend by adding the following in the getActorCommandHelp(commandData) function:
case 'singleSkill': { const action = BattleManager.inputtingAction(); const actor = action && action.subject(); const acWindow = SceneManager._scene._actorCommandWindow const object = actor && $dataSkills[acWindow._list[acWindow._index].ext]; const help = object.description; if (typeof help === 'string') return help; } break;