I don't necessarily agree with what they said. But I do have the thought that for some constantly being in survival mode leaves your brain no time for depression. And it only sets in when you have time to think about stuff. (Disclaimer I am diagnosed with MPD).
When your only focus is on keeping yourself and children and community fed and clothed, you essentially don't have much time to dwell on things, you don't have the time to be depressed because you need to survive. (That's just my thought, and I don't think it's completely accurate either).