It is unlikely that adapting the upper body muscles to be better fat burners is going to help much with your climbing endurance. The intensity for all but lower angle (low 5th class) climbing is too high and hence needs to fueled by glycolysis. This is probably the rational behind the Anderson Bros not even mentioning the aerobic threshold, below which, fat is the primary fuel. On top of that, it is common for the upper body to contain a higher percentage of FT fibers than the legs. Makes sense right? We evolved to walk around for hours on just our legs. Quadrupeds have equal ST fibers in front and hind legs.
ARCing is meant to be done just below the anaerobic threshold so will be using significant amounts of glycogen for fuel. It is going to be a more productive session if you have some carbs on board for ARCing.