A magic number is just slang for a number which has no obvious reason for its value.
I literally can't discuss this because I have no idea what it's saying. It seems obviously false to me when we factor in the land used for their food production. In the heaviest meat eating/producing countries only a minority of calories produced are from pastured animals. Most cattle are from factory farms involving feed lots, pigs/chickens/fish are fed crops grown on arable land.
Like ~75% of the world's soy is grown for animal feed https://ourworldindata.org/soy and soy is a massive crop so it's hard to imagine where all this saved land comes from. What are you comparing against?