If I'm reading the methodology correctly, the paper is mainly comparing the relative findings within each study. (They do have some other comparisons that don't, yes, but they are mainly looking at relative numbers where each is computed with the same methodology)

Our focus on the percent change from a diet switch relative to the environmental impacts of the baseline omnivorous diet described in each study, makes the findings comparable across papers. Within each paper, the environmental impacts of one diet are comparable to those of another diet because these are expressed as a function of calories provided, taking as a benchmark a requirement of between 2000 and 2700 kcal/person/day

They then look at the distribution of the relative change figures. The entire range looked at here is lower emissions

We can also look at non-review studies as well. Here's one comparing emissions of farming types more directly

The aim is to compare the environmental impacts of different diets with different levels of animal product consumption, while accounting for the type of farming systems (organic or conventional) of the food consumed.

A positive link between animal-sourced food consumption and total environmental impact was observed in this large sample of French adults. By far, omnivorous had the highest-level of greenhouse gas emissions, cumulative energy demand and land occupation while vegan diets had the lowest

We found that a 100% organic omnivorous diet exhibited higher environmental pressures, suggesting that following an organic diet without changing towards a more plant-based diet is of little help, at least as regards the studied indicators

the vegan diet, whatever the indicator considered, remained less resource-intensive and environmentally damaging than other diets

https://www.sciencedirect.com/science/article/abs/pii/S2352550919304920

I know it’s popular but believe it or not, the US isn’t the cause of all the world’s problems.

Only 23% of ag land worldwide is used to grow crops for direct human consumption. That’s lower than the US number by the way.

The fact remains that if you actually care about reducing farmland and fertilizer use, you’d go vegan.

Or was I right and that was just a throwaway comment meant to make you feel better about your habits?

That's a cute fairy tale.

In the real world, over 5.6 million tons of nitrogen are applied to corn (40% of which is feedcorn, on top of 40% for ethanol. barely any for us vegans!) each year through chemical fertilizers, compared to a mere million tons of nitrogen from manure. A good amount is coming from cattle, like you said, but the reality is that the clear majority is artificial.

And regardless of whether it's natural or artificial, nitrates then wash into the rivers and waterways causing algae blooms, fish die-offs in rivers and lakes, drinking water pollution, ocean dead zones, coral bleaching and other habitat destruction, that shit even gets into the groundwater. In the human body it causes cancers, thyroid disease, birth defects, and probably more we don't know about.

Poison isn't better for you just because it's "natural" 🙄