No doubt you are right. I'm sure the slavers had all sorts of tactics, and opinions about both their tactics and the enslaved, which were wrong.

The author seems to be positing the "easiest" explanation for why a particular opinion took hold among the slavers. Personally, my parsimonious explanation would be groupthink/received wisdom. Which, if it had to have a "legit" origin, was probably a multitude of factors such as you describe.

But, I'm not the author, and was just trying to explain why I don't think that passage is the heinous smoking gun that so many people think it is.