What does the large and strong arm bone really indicate? High "single rep" strength? Or the fact the individual performed thousands and thousands(millions) repetitions with that arm (lifetime of paddling)?

My understanding is bone density & diameter is indicative of ability to handle high physical loads. I.e. "single rep" strength. While not directly related to speed, it is a pretty good proxy for the ability to sprint. I would also give a lifetime of physical exertion as a pretty good basis for excellent endurance.

Another consideration that should not be ignored is technical proficiency. The younger you start learning to perform a skill the better you will be at it. For example, as a solid mid-pack racer I am not very strong, my endurance isn't that great, but I started kayak paddling when I was around 10. My paddling technique is relatively good when compared to stronger, more fit paddler who started later in life so I do OK. I am good at transferring my limited strength to moving the kayak, and i am good a knowing when and how to take advantage of water conditions to assist in maximizing my efforts.

Greg Barton himself relates how, when he was hot off his Olympic win, he went to the Molokai Challenge thinking he was the fastest paddler on earth and should be able to smoke everyone. He got demolished. His skills were not appropriate to win an open water race requiring wave riding and rough water handling. He is still a faster flat water paddler than Oscar Chalupski, but Oscar beats him on open water. Who is "stronger" isn't really the right question as the appropriate skill set is a significant variable.

while I'm sure there were times they lacked sufficient nutrition, the pre-contact Aleut population was estimated as 25,000 seems to indicate that they were able to thrive in their home environment.

Bottom line: I would not bet against a top (ability varies in all populations) Inuit or Aleut paddler of 100 years ago in a race against a modern Olympian. They were physically strong, very fit, skilled paddlers, and it would be a mistake to dismiss their abilities.