The Supreme Court did not render an opinion regarding the usefulness of a short-barreled shotgun, but deciding matters of fact is not a function of an appeals court. The Supreme Court most certainly DID render a binding opinion regarding the law. As a matter of law, the Supreme Court decided that the lower court could not dismiss an indictment for violating the NFA 34 without considering that some arms may not be protected by the Second Amendment. [Note that I disagree with the Miller Court.]
"Reversed", however, means that the decision of the lower court does not stand. The lower court was most certainly NOT entitled to just repeat its prior decision. Rather, the Supreme Court pointed out that a decision regarding the protection of the Second Amendment in Miller's case depended upon the usefulness of a short-barreled shotgun to a militia. There was no expectation that the lower court consider Miller's membership in a militia.
The Supreme Court agreed with one of the prosecution's arguments, that the possession of only some arms are protected, but the Court disagreed with the prosecution's other argument that only militia members are protected.
"A duly interposed demurrer alleged: The National Firearms Act is not a revenue measure but an attempt to usurp police power reserved to the States, and is therefore unconstitutional. Also, it offends the inhibition of the Second Amendment to the Constitution, U.S.C.A.-'A well regulated Militia, being necessary to the security of a free State, the right of the people to keep and bear Arms, shall not be infringed.' [307 U.S. 174, 177] The District Court held that section 11 of the Act violates the Second Amendment. It accordingly sustained the demurrer and quashed the indictment."
As no facts were presented during the appeal, there were no facts to provide the evidence the Supreme Court desired when they said, "In the absence of any evidence tending to show . . ."
To the best of my knowledge, the Supreme Court can't just announce that it wants a case to be heard another time becuase they want more information. Hence they reversed it and remanded it.
Did they disagree? Or were they silent? Big difference.