I don't think that'd be much of a surprise if you read much literature on exercise - exercise improving mood & cognition is a pretty common result.

I knew of the literature you mention (well, not the specific literature, but that's not the point) - which is precisely what caused me to look at it - but I thought other factors would still be more correlative. I would have thought sleep, stress, recreation time, or social time would be more predictive. But those barely correlate.

Or vice versa? (I feel very hungry on modafinil - does hunger cause me to use modafinil?)

I quite intentionally didn't say one caused the other. But I wouldn't expect the primary causality to flow from energy to eating, as opposed to from eating to energy. (Or, yes, from a common third cause)