This comment makes it sound like you read that and just accepted it as true. Surely that's not the case?

When orthodoxy shifts it doesn't mean that the new orthodoxy is right. These things seem to come in 20-30 year cycles (remember the one about how saturated fat caused heart disease? or how breast-feeding was really bad... I mean, really really good?). It occurred to me that maybe one reason for these cycles is the rise of new generations of researchers (it fits the cycle length). The best way to make your mark is by discrediting the theories of the earlier generation. (I'm not suggesting all of science works this way.) This sort of churn is perhaps an indication that skepticism is appropriate. Otherwise you end up with that amusing illusion of how stupid people always were 20-30 years ago.

Nonetheless, the mechanics of brain atrophy (not that I understand them, but I understand how muscles atrophy a bit from wearing a cast on my arm) do seem genuinely helpful in explaining what it feels like to be depressed. It really does feel like something is happening to your brain. And these people seem to think so. No idea if its true or not, but its helpful to have that idea out there.

Incidentally, I like how you're processing this information through your own experience.

Depression as my brain cells dying scares me (motivates me) a lot more than depression as a chemical imbalance..

I've had therapists/counselors tell me that exercise programs are one of the first interventions they try for depressed patients.