It’s not entirely clear to me how that could possibly be the case, when pasture raised animals often graze land that is completely unsuitable for agriculture. They also eat tons of byproducts of agriculture that are not suitable for human consumption.
This thing about how vegetarianism is sooooo much better for the environment is such a meme. Especially when you take into account what a tragedy modern industrial agriculture really is, with its monocrops, deforestation, and so on.