They did not even really declare that. They said "in the abense of any evidence" The Supreme Court hears arguments, it does not take evidence. They sent the case back to the trial court for "futher proceedings".
But by that time, Miller was dead, his co-defendant (in the original case) copped a plea and the original district court judge, who had ruled the law Unconstitutional based on violation of the Second Amendment, gave him probation, which he successfully served and was discharged from. Miller was shot with a .38, but he had fired several rounds from his own .45. Guess he wasn't a very good shot. or more likely faced multiple assailants.
No other additional proceedings were ever held, proceeding where such evidence, and there was plenty, of the usefulness in a militia/military situation of short shotguns could have been introduced. (They are useful in a Naval boarding or anti-boarding role, for cavalry, or in a modern context, vehicle crews. Although that role has been somewhat supplanted by Short stocked Carbines, with what would be illegally short barrels under the National Firearms Act, but in 1939 they had not been. Even the sub-machine gun had not yet really replaced them. Although the WW-II "M3 Grease gun" and the M1 Carbine finally began to do so.
Justanian said natural law cannot be changed..it is fixed. Man can only change his laws..