I think the OT correlation makes a point to question the causal direction b/t depression & intelligence, more pointed than obverse "ignorance is bliss".
Depression: 1st Order - causal inefficacy w.r.t. "locus of control" - general
2nd ||. - ||. ||. Depression itself - specific
As the disease turns against itself, it seems intelligence grows at the expense of praxis. So for as smart as we get, it does not really end up doing anything, other than fortifying the illness.
At it's source, I think depression is actually caused by a tantalizing misconception of what it means to be having the human experience. As long as we continue to force the idea that making a difference involves the causal efficacy to perform a specific task, we lose the deeper connection to the involvement in experience that occurs before any specific intention is even determinately known, let alone completed.
Our experience of freedom has been masked and concealed to only reveal what our "masters" allow us to consider. Then we take it personally when we do not perform according to our now internalized limitations. Has me banging my head against the wall sometimes. It's like a paradox in which I resign myself to getting out of my own way in order to actually approach myself.