I provided the quote for the definition of militia in Title 10:
"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."
It does not define the militia as the National Guard. It defines the NG as a part of the militia (specifically the organized militia).
The "declared intention" is to become a citizen. Not a member of the NG or the militia.