From where I stand, this is a no brainer. It makes good evolutionary sense for activities like gardening to be deeply embedded in our physical and emotional wellbeing. I've always felt somewhat self conscious about how much joy I get from gardening, and from cooking and sharing the things I've grown. However, I reckon the "buzz" you get from growing things, and particularly from growing edible things, is not just an inner-city-hipster-urban-farmer-reconnecting-with-the-earth moment. It's part of our genetics. Of course feeding our families with the results of things we've grown or gathered is immensely satisfying, it is no doubt deeply connected with the pleasure responses in our brains. Because for vast swathes of human history, it's how we've survived.
It's just a hypothesis, a good one I think, and no doubt you could spend time and money on some nifty research in order to collect solid evidence to support the evolutionary links between gardening and human wellbeing and how this can benefit those who are physically and mentally unwell. But for my part, I think it would be somewhat pointless. The bit I like best about this article is where the talk about "sensible and pragmatic use of evidence." Ie. it doesn't cost much and it's not going to have a whole lot of negative impacts, all evidence suggests it'll do you the world of good, so bloody hell, go and do some gardening, it might just cure what ails ya!