Carrying a shoulder slung camera bag for a few hours I will never go for that style. The uneven pain it caused was just amazing for such a light load.
While I have no way of knowing why or if Cody might prefer a messenger bag/haversack over a backpack, there are perhaps some advantages for his style of survival.
For example a messenger bag/haversack and be adjusted quickly to ride more on the back, side or front depending upon how you want to distribute the weight. It can be accessed without having to remove it from your back. If one is foraging, you can pull the bag to the front and place or remove what you want inside quickly. It can be moved from one shoulder to the other easily. It can be carried in a tumpline fashion.
The versatile nature of how a messenger bag/haversack can be carried makes me think that it would make a good choice for a minimalist survival style such as Cody’s.
Pete
I Cant honestly see one good point to messenger bags that gives them better use over a small backpack of the same rough size. Backpacks can be slung over one shoulder in front for easy access just like a messenger back and when foraging Im sure the messenger bag would more get in the way then anything when leaning down/forward. Ive Never had them "just stay put" when moving around like a backpack style pack does.
As For distributing weight where you want it, Ive never thought in my life when carrying something "Gee This uneven weight to my right would feel much better if it was uneven on my left". Even while Carrying it towards your back your now using one arm to hold it behind you locking it up from use and its still resting its weight on one shoulder.
Id much rather have a Small camelback backpack then a messenger bag. I very much believe the messenger bag style for him is all about "image" more so then anything. How Much of a barefoot nature loving person would he look like with a high end expensive sports water bladder pack on?