He's a Triathlete. For Men's Sports like any real code of Football eg Rugby or Rugby League, you need the big movements to develop the ability to collide with impact and to not get smashed. Even for a trans gender sport like Aussie Rules
you still need strength for jostling, grappling and collision. Sit ups and Leg Raises just won't cut it.
Players like Stephen Silvani, James Herd, Paul Roos, Cameron Smith etc who don't look well developed would still be naturally very strong.