Yes, it's like when they say about modern movies or TV shows: "Based on a true story!" to give it an air of gravitas it wouldn't ordinarily have, but it could be that that one sliver of truth is simply that a guy named "Joe" really did walk around the corner that day ... and the whole, lurid murder Joe witnessed was totally made up.

Personally, I focused on the illness or meningitis angle first because the focus of my blog is on how depictions of disability in folktales are just as based on real-life experiences as any other depictions in the stories (such as ploughboys and scullery maids), and not simply (as they are often treated in folktale analysis) metaphors for psychological states.