I question whether it is a matter of not able to generalize from known examples, as I suspect that most of the people who do well on the corresponding social questions did not know the answer beforehand, but nevertheless found their way to the correct one.
The article you link to seems to support this view: "Cosmides and Tooby argued that experimenters have ruled out alternative explanations, such as that people learn the rules of social exchange through practice and find it easier to apply these familiar rules than less-familiar rules."
I now wonder if people tend to do better on abstract Wason tasks after being prompted by logically-equivalent social-relations questions, as opposed to being prompted with additional abstract problems (without being told whether they gave the correct responses to the prompts.)
My guess at what might be underlying these outcomes is that people clearly see the purpose of the social rules but not of the abstract ones, and our intuition is attuned to assessing how various social scenarios will play out, taking into account the motives of the actors (this might seem to be the same as what Cosmides and Tooby claim has been ruled out, and maybe it is, but I think what they are ruling out is us having learned various schema to be applied mechanically in the corresponding social situations.)
Alternatively, could it be something as simple as this: having an understanding of the social rule's purpose helps the subjects avoid confusing conditional and biconditional rules? At least in the English language, acceptable usage can be quite ambiguous in this regard.
https://en.wikipedia.org/wiki/Wason_selection_task