> That is, the medicine did not contain chacruna (psychotria viririds) as it does today in order to bring about visions

Indeed, but harmal alkaloids - those contained in b. caapi - are not only selective, reversible MAOIs but also full on psychedelics by themselves, as anyone having taken a significant amount of them (usually either through b. caapi or a peganum harmala seeds tea) could tell you. You get "visions" with them too.