If it was a revolver he would have simply pulled the trigger again (which he did, apparently) to get to the next loaded cylinder (which he apparently didn’t).
First of all, whenever I'm open carrying a revolver say in the woods with an exposed hammer, I keep the hammer down on an empty cylinder. Always have. Always will.
That having been said, the shooter in this article would only have quickly pulled the trigger if he had the presence of mind to do so. Obviously, there was some pause. He expected the gun to go off. It didn't, That startled him and he spent a moment or two thinking about why the gun didn't go off before he decided to pull the trigger again.
Many years ago, I recall a robbery in which the robber pointed the gun at the clerk, pulled the trigger but the gun didn't go off. Puzzled, he looked down the barrel, pulled the trigger and the gun went off killing him. I thought it was hilarious. Maybe a year or two later, I heard of the same thing happening again in a different robbery. Just because young punks have guns doesn't mean they know anything about how to use them.
Furthermore, whenever a semiauto is involved, the media make sure to mention it. Curious that no mention was made as to the type of gun.