What you miss is that US Code defines militia as the explicit militia that you mention, but also the implicit militia. That implicit militia includes others not in the national guard.

It's also worth mentioning that most states have similar language to the second amendment, including states whose constitutions were implemented prior to the bill of rights, lending to the credence that this is a right of the people, and not of the state. The 10th amendment was supposed to be for the rights of the states.

https://www.law.cornell.edu/uscode/text/10/246