A magic number is just slang for a number which has no obvious reason for its value.
Which number is a magic number to you? I thought I was clear in asking that question.
It seems obviously false to me when we factor in the land used for their food production
I'm not sure what you're saying here. Cattle and most livestock can graze on marginal land. What land would be used for food production? Here's the land-use breakdown.
That's not an accurate statement to the reference. 75% of soy crop is fed to animals. That's a very different reality. It still jives with the 86% of feed that is human inedible. How? Because a high percent of the soybean crop is inedible to humans, and there's been a huge influx (your link agrees) in demand for soy products in general. That soy waste a cheap option for feed. The alternative is burning..... but we cannot continue down this line without dropping the land use topic. 100% of the marginal use livestock diet COULD come from the marginal land. If we didn't need to get rid of this other stuff anyway.