The argument is not concerned with the “danger” of these drugs in the food supply. It is only concerned with the increased likelihood of the drugs in the food supply

While this provides a way to limit the chance of cross-pollination, it does not say whether this will actually happen. It is phrased as if-then, not something that will definitely occur.

Just because the drug-producing plants are not among the largest portions of the general food supply, it does not weaken the argument that they are more likely to enter the food supply. Maybe that likelihood is not as high, but it is still there.

This suggests that if cross-pollination were to occur, the harvested parts of the plants would not contain the pharmaceuticals and thereby not enter the food supply.

Although this suggests that crops altered by cross-pollination could be identified, it does not give enough evidence to assume that this will decrease the likelihood of drugs entering the food supply.