That's a misrepresentation of what he was saying, don't know why you got the upmods. A captive-bolt stunner is a quick strike that stuns. "beating" something is protracted causation of pain. You're not 'filling in' details that have been left out, you're creating a new situation out of whole cloth.
"A captive-bolt stunner is a quick strike that stuns."
If it's successful, yes. They often aren't; it's not hard to find undercover video of "stunned" animals in clear, loud pain.
Let's take this whole what-if to the extreme:
If I had to be turned into a cow and slaughtered and was given the choice of a random secular slaughterhouse that would use a stunner and a random kosher slaughterhouse, I'd absolutely choose the kosher option. I'd have a better chance of not being injured and mistreated on the way to my death, and I'd have a better chance for a clean, quick, and minimally painful death.
If you had to kill your pet dog by cutting its throat, would you rather beat it unconscious first?
Anything can be black and white if you leave out enough details. And, you can even decide which side is which with your choice of details.