I assume this is the paragraph you're misunderstanding:
It could have made no difference to the trial of a charge of involuntary manslaughter as to who loaded the gun or whether the person handling the gun was sober or drinking or whether he had loaded the gun while drinking intoxicants. All that it is necessary to establish for involuntary manslaughter by the use of a loaded firearm is that a defendant had in his hands a gun which at some time had been loaded and that he handled it, whether drunk, drinking or sober, without due caution and circumspection and that death resulted.
Read the paragraphs before:
The second assignment of error is to the effect that it is necessary for the court to instruct the jury in the exact language of the particulars stated by the district attorney at the opening of trial, as follows:
"That the specific act or acts consisted of carelessly handling a loaded firearm and in particular that the defendant loaded the gun, that he handled it without due caution and circumspection while he was drinking, and that he drank intoxicants while he had a loaded gun in his possession." (Emphasis by the court.)
All that can be required of the court's instructions is that they properly give to the jury the essential facts which must be established beyond a reasonable doubt before the defendant can be convicted.
In stating the essential elements in instruction No. 5 the court properly informed the jury as to everything which must be established in this case by evidence beyond a reasonable doubt to convict defendant except as to the gun, or pistol. In instruction No. 8 the court particularized as to the handling of the gun and expressly told the jury that it was required, to warrant a conviction, to find beyond a reasonable doubt that defendant handled the pistol without due caution and circumspection and that his so doing was the proximate cause of the death of Eva Devoll.
In the appeal, Gilliam tried to argue that his conviction should be overturned because the jury instructions didn't include the full charges as they had been set out in the charging documents - and instead, the jury was given the statute. The text you refer to is dicussing those instructions, not somehow disallowing defenses.
The part you're ignoring - and that the Court did not ignore - is the last part: Without due caution and circumspection.
The question is how much caution and circumspection are due, in the circumstances.