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).
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.
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.
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.