At first read, I thought it could be an interesting point of view. And still do think so. However, I think this article is kinda biased in the way it does not focus on the fact that the study is just for the U.S soil.
The type of land that makes the U.S. soil is not the same as everywhere, thus the crops are not the same.
My second point is that one of the flaw I see in this study is that it might be right to say that we may not exploit as much soil for vegan diet support as for dairy-friendly vegetarian diet support. But what the study does not explore is the environmental cost of those specific soils. It is well known that there is a lot of deforestation to actually support the feeding of cattles and co. What about this cost?
Plus, it seems the study only speaks of land you can use. But in meat/egg/dairy consumption, it is not only a matter of crops. There is also the land used for gathering the cattles, the effect of the breeding (water pollution, greenhouse gas, land destruction, lost in biodiversity, …)
There is probably other points that should be fixed but I think that is fine enough already to make my point.