Just finished reading this article in the NYT. Particularly the part about how Darwin's better work might have been done during bouts of depression because it was only his obsessions in the area of science that gave him any pleasure in life.
So maybe as simplified models is :
a person is prone to depression, which a sort of obsessive thinking about woe and personal problems. Should they be fortunate as Darwin as to have a different direction to point their obsession, they can disrupt one dysfunctional obsession with another. Finding a suitable obsession is not straight forward matter, it needs to be inherently fun and have some depth to it. Ice cream is inherently fun, but eventually the containers of Ben and Jerries are all empty and the stomach is full. Conlanging (and learning foreign languages in general) on the other hand, has enormous depth, especially compared to the time available to any single conlang designer. Constructing the rules and more over the corpus for a language takes forever.
So anyhow, maybe the link between depression and toki pona is mostly in that toki pona is easy enough to get obsessed with to disrupt a dysfunction dwelling on ones own ike pilin.