This seems more like someone's specific theory of federalism more than anything widespread or accepted by a large number of constitutional scholars.

There's tons of overlap in state powers since basically the 1930s and the expansion of federal power. Some are simple like military responsibilities shared between the Executive and Congress for commanding and funding and provisioning armies, and state's militia powers. Other stuff is more complicated and has changed overtime, like the expansion of commerce clause power into what was traditionally seen as a state's responsibility for providing for the "common weal", which meant any kind of safety, or health regulation or policing powers. Some stuff has changed b/c of amendments. Citizenship in the U.S. as a whole was determined by states until the 14th Amendment, but it was a joint issue b/c the citizenship in the state would make you a citizen of the US and able (hypothetically) to use federal enforcement of the privileges and immunities clause.

There's a lot of overlapping areas too, like the power to tax. Basically you can scan through Art I. Sec 8 and count tons or areas where the powers are overlapping. States and the Feds can both borrow money (but this is complicated b/c of issues of state banks issuing paper money and issuing bonds), both can establish courts and their courts can hear both state and federal matters in some areas (concurrent jurisdiction). There's other stuff like voting that was mostly a state power but has become more federalized b/c states failed to really protect the right. Some stuff varies from state to state, like my state has protections for free speech that go beyond the 1st A. and other states have gone the other way and have tried to limit stuff like free exercise of religion or establish religion, which used to be a state right, so that would indicate an overlapping power if a state can expand on it.

I read a fair amount on the Constitution and an advanced degree on the topic and I can't remember hearing the 6 powers theory and thinking through the big popular books on the Const, like Akhil Amir Reed's books or Chemerinsky's, I can't remember it ever being mentioned, even as an argument they counter.

I could easily be wrong b/c there are literally 1000s of books on the topic and some that were popular 2 centuries ago are basically unread today, so it may have been an older idea.