I do not find it surprising that a smaller boxer who may be the best in the world pound for pound does insane amounts of running. Many boxing trainers emphasize that road work is the most important part of training once a guy learns the fundamentals of boxing. More than a few boxers have shown that they can transition to running good marathons without much trouble.
I also find it interesting that so many boxers seem to take a HADD approach to their running... they jog it at what most would consider an insanely slow pace relative to what they are capable of. Maybe they figure this causes the least interference with their training while still providing the results they seek.
I would not be surprised at all if he is telling the truth.