Great link including the analysis.
I did a quick analysis of the bending stresses on the trunk and the rebar and came to the conclusion that the rebar should be 40% of the diameter of the trunk based upon the modulus I could find for pine. The 3/4" (#6) rebar was obviously too small based upon this, but I doubt that 1.6" rebar is necessary (I guessed the trunk as 4" diameter at the cut). Therefore, the modulus of the live trunk must be less than the published values for pine. It will be interesting to see what size rebar works - my guess is that 1" (#8) may work and that 1.25" (#10) will work.