Most deforestation of the Amazon is done to create more land for cattle, to give you one example. It is by far the largest driver of tropical deforestation, and almost all global deforestation is tropical. Grazing land that can't support anything else is by definition not included in statistics on deforestation or loss of biomass and biodiversity.
https://ourworldindata.org/what-are-drivers-deforestation
It's true that a lot of cow feed is stuff humans won't eat, but it's not true that it would all be wasted if we stop raising cows. We can grow less of it, we can throw the scraps in anaerobic digesters to generate power, we can simply feed it to other animals, as a last resort we can use it for compost.
I'm pretty sure vegans are less than 1% of the world, and I don't think they have some special influence over the IPCC and most climate scientists on the planet.
This is typical vegan misinformation to appeal to people who are concerned about global warming (as we should).
Land that's used for grazing is typically land which can't be used for other purposes. People don't grow grass on flat fields of black soil because it doesn't make economic sense.
Additionally, animals are also fed the leftovers that people don't eat. That is, when you grow corn, we eat the grain, and the rest of the plant (leaves and all) is given to animals. Without livestock that food would just be wasted.
These are just some of the many ways vegans mislead.