They are probably using large psychoactive doses of Brugmansia. Psychoactive doses are very dangerous.

1-3 seeds of Datura stramonium is no where near a psychoactive dose. I don't think it's going to impact the trip much on it's own, but will probably increase the psychoactivity of the harmala alkaloids. That's my guess anyway. Datura inoxia is more like Brugmansia than Datura stramonium. My feeling is that Datura inoxia would go better with caapi than Datura stramonium. It has a similar feel to caapi, while Datura stramonium is more of a stimulant. But I believe that Datura stramonium seeds would potentiate the effects of the harmalas more than Datura inoxia seeds will. But without SWIM ever testing any of this, it's all pure speculation.