They didn't rule that it had no military use, they just ruled that they couldn't take judicial notice that it did, and remanded the case back to a lower court.
No evidence can be introduced in a Supreme Court Appeal, they must work solely on the record of the case. There was no record in the case of any testimony or evidence presented that a sawed-off shotgun had military use.
More to the point, I think, is that there was no record of any testimony or evidence presented that a sawed-off shotgun had NO military use. The burden would have been on the prosecution.
The alternative is to believe that failure to defend oneself adequately is grounds for remand from the Supreme Court or that a person who keeps and bears arms is guilty of a crime until proven innocent. The latter case applies to Kalifornians today.
Which is not surprising since there hadn't even been a trial yet. The lower court threw the case out on a motion from the defense, based on the unconstitutionality of the law. Then there is the fact that no arguments for the respondent (Miller, it was the government that appealed) were presented, either written or oral. No NRA "Friend of the Court" brief. Nothing. Nada. Zip (The Court no longer allows that to happen, since it's a clear violation of due process and the right to an attorney) The argument could have been made there that the requirement for the weapon to have military usefulness was faulty based on a simple reading of the second amendment, which contains no such restriction, and upon the history of its enactment. Part of the reason for Miller not being represented at the SC, was that he could not be located! By the time case came back to the lower court, Miller was dead, and Layton, the other defendants in the original case, had copped a plea to a lessor offense. Thus no trial was held, and no evidence presented about shotguns or anything else. The Miller case is the last time the Supreme Court directly addressed the scope of the second amendment, and it's a mighty thin reed upon which all "modern" gun control rests.