Their point is that if plants can suffer, and assuming we still want to eat, less plants die or are maimed on a vegan diet than on an omnivorous diet because livestock eats plants too and the conversion to meat is inefficient.

That means vegan diet is the way for less plant suffering even though you eat them directly. In fact it is because you would eat them directly.

Then vegans are worse, they make plants suffer more,since they’re not as adapted as herbivores for plant consumption.