i see your point, but it doesn't mean that they are identical. you would need to do another test - one that compared people who were tired through some task that was not considered "hard", and those that are tired through something that is considered "hard". if the two groups behaved identically, then you would be right. but you are assuming that result in what you write.

alternatively, it might even be the same task, but two groups that have a different attitude to it (some might consider it play, others work).

(i suspect you can also continue down your route, re-labelling things; for example, you might say that people only calls things "work" if, for that individual, they are "theft-inducing". at that point i am not sure if we simply agree to disagree, or if the argument becomes whether you are still learning something new over the original experiment)