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.
Native plant/ecosystem restoration and gardening especially where it intersects edible plants. Also especially around the 45th parallel in N. America.