There are no studies like that, though. It's very very hard to study how a group of people suffering from a disease evolves without treatment, both from an ethical perspective (you can't ethically force people not to seek treatment) and from a practical perspective (it's hard to find a significant cohort of people who recognize they have a disease like depression, do not want to seek treatment for it, but are willing to participate in a clinical study). This is especially true for depression, which manifests most of all as a lack of motivation to do anything in those who suffer from it (often with anxiety at the thought of doing new things).

So no one really knows, in a scientific, quantifiable sense, how people who suffer from depression evolve without treatment.

In that case it's unblinded and placebo could be the patient giving the interviewer what they think they want to hear.