The definition linked above does seem to generally imply (and in some cases explicitly state) that depressive episode/disorder symptoms better explained by some other diagnosis or situation (like schizophrenia or bipolar disorder) should be taken as further symptoms of that disorder/situation rather than a separate diagnosis of MDE/MDD. I guess the question is whether or not symptoms induced primarily by external stimuli (as opposed to when the symptoms are expressed as irrational thoughts regardless of external stimuli) ought to be under a similar banner of "these are symptoms of $SOME_OTHER_CONDITION rather than depression". My (again: non-professional) opinion leans in favor of "yes" being the answer, since it likely has different treatment implications (remove patient from symptom-causing stimuli v. prescribe cognitive behavioral therapy and/or medication to patient), but that's the thing about brains: nobody really understands how the brain (and thus the mind) works, and thus it's unclear whether or not such a distinction would actually be useful.
It seems likely that both external stressors and internal thought processes can both be the cause of depression (and in many cases, a combination). CBT addresses the internal thought processes, but doesn't do anything to address external factors. My wife underwent CBT and took antidepressants, but they didn't really cure the depression. Quitting her crappy job fixed it for good.
Do the other factors in that particular condition still apply? Namely, "the symptoms persist for longer than 2 months or are characterized by marked functional impairment, morbid preoccupation with worthlessness, suicidal ideation, psychotic symptoms, or psychomotor retardation."?
Maybe not in those words, but the DSM does appear to distinguish between a depressive episode and e.g. bereavement: http://www.mental-health-today.com/dep/dsm.htm (I don't think this is the actual text, but it's the best I can find without a copy of the DSM-IV-TR on hand).
IANAPsychologist, but as an ordinary person, the article's distinction between sadness and depression seems reasonable and intuitive enough for a layperson to understand, and that's a step in the right direction in a world where laypeople (including people who might very well themselves have depression) rarely understand anything about the actual nature of depression.