First of all, the claim itself can probably be traced to the follow sentence from Essens' abstract:
From the systematic errors subjects made in reproducing temporal patterns, it is concluded that an accurate internal representation will be arrived at only if the temporal structure of a pattern enables an organization in which hierarchical levels relate as integers with prime factors 2 or 3.While this sentence is, indeed, consistent with the experimental data, the paper concludes with a "General Discussion" section, which, in turn, concludes as follows:
The research reported here restricts itself to conceptual structures used by subjects from a western culture. In other cultures, different conceptual structures might be used. Data across cultures could provide insights into the universal nature of our results.Exactly. Essens was careful to establish the limitations of his conclusions based on the limitations of his data, and it is more than a pity that some of those who read those conclusions did not respect those same limitations.