I don't think there's a specific point where they explain how, but considering Jin's housekeeper knew about it and the flowers are everywhere, I'm guessing it being poisonous and how to extract the poison is reasonably common knowledge. Though, you can heal from their version quite easily and it doesn't kill as fast as Jin's, so I'm guessing they don't know how to make it as potent as you do.
I just find it weird how it's suddenly everywhere and Jin is blaming himself.
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PSN, and GT: Waluigi1 Switch FC: SW-6848-3841-9099