That's misleading. Lifestyle changes can have a significant positive effect, but that's also misleading, because you have to account for the often insurmountable inertia that old behaviors have.
Furthermore, the most reliable predictor of manic depression in later life is more than 2 traumatic experiences before age 7. I might be misquoting the figures slightly: it might be 3, not 2, or 10, not 7. However, the point I'm making is that in many cases saying that depression is caused by erroneous lifestyle is plain wrong.
I agree with you on medication, it has it's place, but it's hardly the end of the story. I personally don't take it, because I feel that my case is more suited to a different therapy, though I feel a bit silly about never having tried it. I think that one of the best things you can do is try a lot of things, to see what works for you.
It must be nice to know so much about everyone around you, without needing to actually obtain any facts at all.
Is it possible that changes to exercise, diet, time outdoors, or latitude of residence could help in some cases? Maybe.
Is there anywhere near enough data to attribute a "vast majority of cases" to these causes?
No.
That's not to say medication doesn't have its place. It's that suggesting we eschew all other measures in favor of medication is ridiculous.