I think he meant that the process wasn't explained as 'removing poison' but as 'this is how we do things' or 'it's to appease the spirits'. If the poison was something that's slowly acting (years) 'rational' way that would remove 'superstitions' would poison everyone.

Of course, knowing the real reason is preferable, but not always possible, especially in a premodern setting. Before the discovery of prions nobody really knew why cannibalism is dangerous, yet it was a taboo in most of the civilized world.