Isn't this the cloudy day tanks problem of yore?
This is the obvious explanation for their initial results, and one of the first things I said when I saw the preliminary results: "how do you know there isn't some subtle association between 'eagle' and $arbitrary_small_integer that you are just ignorant of but the superhumanly knowledgeable LLMs have learned about?"
But then the later experiments rule that out, in part by showing that it doesn't transfer across different models (ie. initializations).