This is asked further down the linked thread. I can't link on mobile so I'll quote /u/race-hearse here:

Yeah. I think a way everyone should reframe the concept of depression is that it's a disease causing compulsive ruminations. That's all it is. That's what medication will help you treat: compulsive ruminations.

Rumination means something like "focusing on the source of your despair" and compulsive just means you can't help it. The last I heard about the research about it is that depression is likely caused by someone having something to cause some despair, and either that despair was so great or you're genetically predisposed, that when that despair kinda becomes an unbreakable pattern, you've got depression.

... Omitted personal anecdote for brevity ....

Now, as for what that's doing in the brain. It's changing your behavior to avoid activating your amygdala for extended periods of time, which is what supposedly triggers and brings depression back to begin with. Supposedly prolonged activation of the amygdala can kinda mess up other areas of the brain and then just keep it stuck in the on position (and that's depression.) So if you can avoid activation of the amygdala you can prevent it.

There's more on the amygdala in the post.

I happen to find CBT only useful as much as it would be for helping people cope with any chronic disease or tough situation, and disagree that all depression is a disease that causes rumination. However, this is how it manifests for many, and why talk therapy helps them in concert with antidepressants.

My model of my clinical depression is even more materialistic broken brain, because I only get relief from medication. However there is another useful comment in that thread about how what we call depression might be two different mechanisms with similar symptoms and therefore need two different treatment routes.