What kind of surgery was each patient going in for? To me, these results speak less about ketamine and more about how solving longstanding health issues with surgery is highly likely to improve depression. Reduce daily suffering noticeably and people are likely to be less depressed.

Also, how long before the general anesthesia were these patients given either the saline or ketamine? Most ketamine therapy I have seen involves taking a dose of ketamine during a special session with a therapist, or given to people as an inhaler for panic attack treatment. I'd wager if they didn't get time to sit with the ketamine, introspect, and dissociate it won't actually help their mental state.