The claim here is that plants have agricultural runoff so it's environmentally damaging to be vegan, whether because of the nitrates used in fertilizer or pesticides
It takes far more plants to feed an animal than it does to eat plants directly. This means that one increases the amount of runoff by consuming animal products. So if you are truly concerned about agricultural waste, you should go vegan.
Not only does consuming animals increase the number of plants that are required to feed everyone, but also the raising of animals is resource intensive and directly contributes massively to the waste produced. For instance, the meat in a burger is highly water intensive, with cows drinking up to 30 gallons of water a day. Where does that water go? About 8 gallons becomes milk and the rest has to go somewhere, as even non-productive cows take up to 20 or so gallons a day. It should be obvious that it ends up washing off back into the environment.
So it not only takes more plants to feed an animal than to eat the plants directly, but one also has the additional polluting contribution of animal agriculture.