WHAT???? That doesn't sound right. No human on earth can life a real log and swing it like a bat since they weigh several hundred pounds. If you meant he grabbed a big branch and hit the bear with it that would almost never kill an adult male bear since they are very tough. It could scare him and cause him to run away or just piss him off where he would attack.
It takes more than that to kill an adult male black bear.
Do you have a link or something about this story??
Just like a human, if the trajectory, force, and angle is just right, it absolutely could kill a bear. And by all accounts, it did exactly that.