Seven Reasons Why Most Major Depression is Probably Not a Brain Disorder
Note that they say "most," not all. Bipolar depression sure seems like a brain disorder of sorts, as do depressive symptoms related to peripartum, hypothyroidism, some GI cancers, etc.
To my mind, it's best to think of "depressive symptoms" rather than of a unitary phenomenon. Many acute depressive events I have seen are, in fact, reactive to events and could be viewed as grief.