In the theory of depression as an attractor state in a dynamical system[1], exercise would be one of many small cures for depression. To cure depression, one would likely have to combine several small cures, none of which would work on their own. For example: diet, exercise, amount of sleep, timing of sleep, pharmaceuticals, behavioural activation, cognitive distortions, social life, life circumstances, as well as any physical health issues which could be contributing to depression.

I no longer have depression, and it was a combination of small factors which did it for me.

1: https://astralcodexten.substack.com/p/ontology-of-psychiatri...