Actually, what it shows is both that getting ketamine while under general anaesthesia is roughly as effective as when taken without anaesthesia (40% here vs 45% in [1]), and as effective as just taking anaesthesia (their placebo treatment).
I see. But this seems to cast even more doubt onto the use of general anaesthesia as an active placebo. I mean, usually people waking from it don't feel great because they're very ill or had major surgery. Maybe in the abscence of things like that it could have some antidepressant effect.
Things similar to this have been used as treatment in the distant past of psychiatry as well, when ethics was nowhere to be found. Induced comas and such.
To be precise, the anaesthesia is part of the masking procedure, the placebo is a saline injection that was administered the same way as the ketamine (in practice the ketamine was simply diluted in a saline dose that is then given to the patient).
Of course, getting all those anaesthesia drugs into you could mess up the body chemistry in all sorts of ways that make the ketamine not work, but it is still very surprising that the net effect of this big cocktail of substances is that you get basically the same outcome regardless of treatment method.
Anything that puts you under and also inhibits REM sleep, can have effects against depression. REM sleep is important for memory consolidation, and consolidation of memory is linked to depression in that the reliving and reconsolidation of painful memories is thought to be involved in at least some types of depression.
This is why sleep deprivation can have an acute antidepressive effect, and also why cannabis can have a similar shorter effect in some cases. Usually when there's no tolerance.
I think it's quite possible indeed that something similar happened during this study.
Things similar to this have been used as treatment in the distant past of psychiatry as well, when ethics was nowhere to be found. Induced comas and such.