The paper is from 2013. The number of animals in factory farming were ramped up since then.

Then: She says crops not crop calories.

“From the 41 crops analyzed in this study, 9.46 × 1015 calories available in plant form are produced by crops globally, of which 55% directly feed humans. However, 36% of these produced calories go to animal feed, of which 89% is lost, such that only 4% of crop-produced calories are available to humans in the form of animal products. Another 9% of crop-produced calories are used for industrial uses and biofuels[…]”

55% + 9% = 64% = 2/3

So you’re correct with the 2/3 crop calories(!) if we count the 9% biofuel/industrial stuff with it.

But why is the following?

“According to a 2011 analysis, 75% of all agricultural land (including crop and pasture land) is dedicated to animal production.”

I’d suppose it’s because of this reasons: A. Pastures need a lot of space which for which often woods or rainforest are burned and biomes are destroyed. B. Probably the most eaten vegetable (potatoe) is very energy dense and has a lot less waste by-product and therefore needs a lot less space than animal feed like soy, wheat or corn.

This is everything I need to know from your paper to say that the system of animal agriculture is fucked, wasteful and destroys nature and our health (not only because of the pandemics it causes):

“Put another way, shifting the crops used for feed and other uses towards direct human food consumption could increase calories in the food system by 3.89 × 1015 calories, from 5.57 × 1015 to 9.46 × 1015 calories, or a ∼70% increase. A quadrillion (1 × 1015) food calories is enough to feed just over 1 billion people a 2700 calories per day diet for a year (which is 985 500 calories per year) [1]. Therefore, shifting the crop calories used for feed and other uses to direct human consumption could potentially feed an additional ∼4 billion people.”

Pastures can be regenerated, btw: https://www.reddit.com/r/farming/comments/1ds3fvh/how_to_turn_pasture_into_a_garden/

Edit: The latter is what the “Vegan Land Movement” is doing very successfully by rewilding pastures to wild lands again where a wide variety of wild life is finding a refugium now.