Well special conditions do have to be considered. Though I believe getting fit does improve mental health (or at least overall health), I think stimulation also play a big role. In the case of quadriplegics for example, it'll all have to be mental. Somewhat same with anorexics. With those with anxiety and phobias would likely benefit from exposure therapy first.
most of these studies are not aimed at proving that exercise is the cure for depression. meaning meds and therapy are not biologically needed.
but instead done to make it financially cheap to medical budgets and medical insurance underwriters by saying people should go for a walk(free) as oppose to being given meds/therapy.
alot of the medical industry customers (hospital/doctors/insurance) that pay the bill have already tried to justify not giving people the medical support the person actually needs and instead find viable free options that people should do. thus making it the patients fault for not trying to better themselves.
only problem is depressed people being blamed when the depression itself is preventing rational thoughts of bettering ones self, just snowballs negatively.
patients ned actual support in many cases. not told that they should just walk more and drive less
EG people with back pain just told to go to local 7-11 and buy some ibruprofen. yet what they actually need is a chiropractor and surgury
and now mental health has become a subject that was undr funded they want to pretend they are solving it, bt without costing them the real costs to actually solve it
EG some people soo depressed they resort to illicit drugs to escape their reality.. being chased by cops and looking over shoulder and racing across town to get the next fix is very exercise heavy.. yet.. guess what its not cured the underlying depression
oh and homeless people that live on the streets and dont have cars thus have to walk everywhere.. they are not the happiest people. so fresh air and exercise is not the sole solution