I think it's BS. I'm very much a "live in the present moment" person (most of the time), but I still get depressed. Sometimes in the present moment, my brain is telling me everything sucks. Sometimes that's true. If depression is living in the past, then why do people who had relatively healthy environments from birth to onset of depression get that onset of depression?

Because life is complex. We all have our vulnerabilities. At some point some/most reach our breaking point and those vulnerabilities are exposed. Those are symptoms of mental illness (though not necessarily a full blown diagnosable disorder). Those vulnerabilities can be depressive feelings, (hypo)mania, psychosis, anxiety, dissociation, self -harm, disordered eating, substance use, other addictive behaviors, etc. It's not because we're stuck in the past. It's because the present is breaking us down and revealing our vulnerabilities.

The past doesn't even exist. It can't be touched. It can't be smelt. It can't be tasted. It can't be seen. It can't be heard. We can remember those things, but those are just memories. It's like watching a movie at a theater. You experience it once, and then it's over and gone. You can rewatch it, but you can't experience it like you did the first time exactly again.