Since he was working from a boat, I'm guessing the photographer fitted the TC in advance without knowing just how close he'd be able to get to the bear. Maybe he thought it was better to err on the side of caution and give himself as much reach as possible?
As for the shutter speed... that does seem extremely high. BUT, having photographed seals and sea birds on a beach from a RIB in choppy waters, I would say that 1/500s would be too slow (I was shooting around 1/1600s and ended up with many not-too-sharp photos
). It's not the photographer's steadiness in this situation that's the problem, but the pitch, yaw and roll of the boat. Although you can compensate for the movement to some extent, it's pretty difficult to remain locked on to a subject and steady in anything other than calm waters, and I challenge any in-lens or in-body stabilisation to deal with the extent to which choppy waters will jiggle a tiny boat around. If you think about photographing birds in flight, you might pick 1/1000 or 1/1250s, and that's for a subject with reasonably predictable movement. I still feel 1/8000s is pushing it, but I can understand going for a high shutter speed.