From the opening paragraph: [He] likens the relationship between mild and clinical depression to the one between normal and chronic pain. He sees both pain and low mood as warning mechanisms and thinks that, just as understanding chronic pain means first understanding normal pain, so understanding clinical depression means understanding mild depression.

In other words, yes, people with clinical depression find that it gets in the way of their daily life. His hypothesis is about why normal, healthy people suffer from low mood / mild depression from time-to-time, and what evolutionary advantage that might give us.