I mostly agree, although I think that the mentioned bidirectional relation between physical activity and depression should be researched more thoroughly. It wasn't hard to find studies supporting it (this one comes from article citations itself, 7th position) or showing no causal relation when the analysis was performed on more specific samples (citations 3, 10 and 16 from the linked JAMA article, possibly missed one or two). Also, I know that it's harder to control for certain variables in psychology, but it could have been more rigorous and attain a much higher resolution, as authors state in conclusions.
Though, I'm conflicted about their FitBit idea. While it would help to gather more information, it would likely just show that competitive/goal-oriented people are more prone to report and feel 'bad mood' or however they phrased it. Either isn't surprising, but it's good to check common sense for biases.
In the end, after dipping my toes into psychology papers for a few days, all I can do is report a good mood for not being a psychologist myself.