There are problems with agriculture. Vegans eat a bunch of foods like tomatoes and broccoli that are low in calories relative to the resource-intensity.
This argument contradicts a lot of other carnist arguments. For one, this claim would indicate that we should only be farming the highest-caloric-content-per-resource foods, which contradicts the claim that the vegan diet is unnecessarily restrictive, which would have one believe that a variety of foods is to be prioritized.
In addition, many nutrients (including protein itself) are only created in plants and are in animals only because they ingest those very plants. So then should one not worry about the vitamins or nutrients to save land or are vitamins and nutrients important? And if one synthesizes those vitamins to avoid growing those crops, isn't that just helping the processed food industry? These claims can't all be made simultaneously.
Also don't carnists eat those foods too? How can one complain on the one hand that growing veggies takes too many resources but then that vegans are missing vital nutrients? So even if this argument were true, isn't this just an argument for never eating a vegetable ever and eating nothing but meat, cheese, and bread? Is that really a diet worth promoting?
It should go without saying that anything that requires some quantity of resources (including land) for growing crops, it requires even more of that same resource for growing an order of magnitude more crops to feed animals to get the same calories.
Think of it this way: if you take crops to feed to a cow, that causes you a 10x drop in the number of calories that are effectively making it into you from the plants. Therefore, you would only break even if you ate 9 parts lettuce or broccoli (plants that are largely calorie-free) and 1 part soy or wheat (plants that are high in calories). Now ask yourself if you see yourself eating a diet that is 90%+ leafy greens (Consider also that bread, [by some metrics, is a further two orders of magnitude more resource-intensive, but we'll ignore that for the moment). Most people are lucky if they see a few percent, so no one is even coming close to the break-even point on this.
So if you are arguing that we need to eat the most efficient crops possible, even given your own assumptions, you should still go vegan.