Dong Li knew this because he saw Chen Jianmin soaking an oleander plant in a scoop of water by the river. Then he used a branch to drop a drop of that water into his food.
At first, he was confused as to why anyone would play with the flower, let alone ruin his food. Although the flower looks beautiful, it is highly poisonous.
It is so toxic that just touching it with bare hands can cause a severe rash. If consumed, it may cause vomiting, abdominal pain, delirium, drowsiness, dizziness, headache, and in severe cases, death.
Therefore at that moment, he thinks Chen Jianmin is stupid as he ruined his precious food. He didn't understand everything until he heard from the villagers that Gou Dan vomited until he fainted after eating.
It turned out that he already knew that his food would be taken away. So, instead of letting Gou Dan enjoy the food, he chose to endure the hunger and use his food to poison Gou Dan.