It's stupid all the way around. The rest of the legal definition of militia from US Law says
(a) The militia of the United States consists of all able-bodied males at least 17 years of age and, except as provided in section 313 of title 32, under 45 years of age who are, or who have made a declaration of intention to become, citizens of the United States and of female citizens of the United States who are members of the National Guard.
(b) The classes of the militia are—
(1) the organized militia, which consists of the National Guard and the Naval Militia; and
(2) the unorganized militia, which consists of the members of the militia who are not members of the National Guard or the Naval Militia.
How old was Miller? Was he in the group described above? If so wouldn't him (or anyone else in that group) posessing such a gun mean that it was "used in militia"?
Since NFA the only reason that people in that group DON'T have those guns is because they illegally banned them. They defend banning some guns because they're "dangerous and unsual" but the only reason they are "unusual" is because they banned them! Of course there aren't machineguns and SBRs and AOWs in every gun owners home... it's almost impossible to get them!