The implication presumably being that JWs are so dumb
I disagree. The peoples temple is a well known cultic group. The reference to the kool-aid is an example of how powerful the forces of influence in the groups can be, that a person may be willing to do something harmful to themselves just because they are told to by a leader in such a group. A person can be influenced to make them think there is only one logical path forward, and so be controlled by the person influencing them. Research was carried out on this by Robert Lifton, and also Margaret Singer, if you want to find more information from google.
These groups are normally focus driven, and so somebody that is 'dumb', or anything else that may impede the momentum of the group, is not recruited. 'The more creative a mind a person has, the more his imagination can be used to control him.'(Steve Hassan, 2000). This would be the exact opposite of your statement.