What I don't like about Jeff Fosters quote is it speaks to only one symptom, and not all will experience the need for "deep rest". I believe depression is more about "unrest". Even for those who sleep 20 hours a day you still feel untested. I have experienced both, sleeping 20 hours a day, never feeling rested, and at other times just feeling isolated and avoidant with a lack of energy but not over sleeping. Then there is all the other symptoms which go along with a depressive disorder which cause physical and mental "unrest".
The other issue I have is the reference to identity. My understanding of depression is that it is a mood disorder not an identity disorder. If there was an issue with identity you would be looking at a different mental health disorder, but I am not a medical professional.