Why would people from a low social level be "wiser" than people from a higher one ? That is not only counter-intuitive, but it contradicts the general trend of research in this field.

The problem here is that "wisdom" is measured as the consideration of the views of others, which is a behavior. And you cannot compare a behavior with an ability like intelligence. Thus what the authors study has little to do with wisdom (unless you consider wisdom as a behavior, but then you could say that ants have a lot of wisdom).

Indeed lower social class individuals tend to be more traditional than higher class ones, and more traditional people act more in agreement with their group. But this does not mean that they have higher abilities at coordinating perspectives. For example, they have lesser negotiation abilities, and, to negotiate well, you have to understand perspectives.

The problem is also that this research is based on unstable presuppositions. We are currently being driven toward a more individualistic society, but individualistic does not mean "self-centered".
There is an important distinction here the authors are completely missing.