On the one hand, there's the formal monopoly on the term that you cite. On the other, one is constantly reminded that "millions of Americans..." have undiagnosed cases of this thing. If not depression, what do I call this condition that grounded me for a decade, and did the same to many people I've known for comparable periods? Getting out of the house to get a formal diagnose wasn't even possible. It may be that I have no idea what it actually is. In that case I'd like to communicate with whoever is in a state like the one I was in, but I don't know what to call them. "Being depressed?"
It takes less than 15min to be formally diagnosed with depression and prescribed SSRIs in the US.
If you're bored you can watch the commercials on TV to learn that it's a chemical imbalance, and then repeat that fact every time a discussion about depression surfaces.
Even a formally diagnosed depression can be many different things. Depression (in the formal and clinical sense) can have myriad causes, symptoms, and cures. Everyone who reads about any of these causes, symptoms, and cures needs to be aware that it may not apply to their specific depression. That doesn't mean we should stop talking about the various causes, symptoms, and cures that are out there, just because we fear giving someone with depression the wrong information.
If you haven't been formally diagnosed, please for the love of god stop talking about "depression". You have no idea what it actually is.