Well, apart from the biochemistry, there is a second layer to the whole story.
Some would call it spiritual, some rather psychological - in the end it accounts to the same thing.

I think that - in addition to the direct effects of the chemicals - the whole ritual with the purging/vomiting symbolism for spitting out "the bad" and so on creates a strong psychological effect. And we today know how powerful our psyche can influence the outcome of therapy or medication, especially when we talk about dependecy or even pcychosomatic or stress-induced illnesses.
So this is an additional way that could indeed lead to a manifestation of the effects that the participants long for.

What would be a really interesting scientific setup would be a study comparing 4 groups:

• one group with conventional medication
• one group with Kambô, but without ritual
• one group with Kambô and the ritual
• one group with the ritual, but some vomit-inducing placebo

Sadly, I don't have the money to run that one (plus it'd be hard getting it through an ethic panel, as it involves giving toxins to humans).