That is insane.
The founders did not want a standing military because they did not trust mercenaries.
So then they wanted all the citizens to be armed and familiar with arms, in order to be capable of being quickly called up for defense of individuals, village, state, or country.
It makes no sense to claim the 2nd amendment was to protect only the states, because the threats came from criminals attacking homes or villages, or foreign attacks on the whole country.
There was never any time states needed an armed militia.
Sure the first part of the 2nd amendment mentions militia, but that is not typically related to a state.
{... 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.
[35] ...}
If the intent was only to prevent state militias from being disarmed by a rogue federal government, then is would not have said, "the right of the people to keep and bear Arms, shall not be infringed."
Instead it would have said, "the right of state militias to keep and bear arms, shall not be infringed."