It's the vegans who have been misled

In terms of land use, the "marginal lands" idea means:

1. Destroying wild land, wild animals, often forests. Most commonly that'll be wild herbivores and various predators. There are even more marginal lands where herders really don't belong, like deserts and riparian areas that are demolished and ruined by the herds.
2. Marginal production. These lands are "terrible", so that means that the plants that grow there, likely some grasslands, also suck. Yes, not all grasslands are the same. Some of them are great for feeding herbivores, some of them suck and aren't even grazed naturally. Marginal lands means shitty lands which means shitty grasslands. In these cases the grassland cultivators try to "improve" grasslands, which means increasing the quantity and the quality of the herbage. This destroys biodiversity.
3. Most importantly, the animals are still murdered.
4. Since the "production" is so low, that means very little animal meat to go around. All that effort, all that propaganda, for very little meat/wool/milk. Which means that it's mainly for rich people, which will cause all sorts of trouble, like what you see in the Amazon forest.

This also assumes the only things worth growing are crops that are easily harvested by mechanized means.

You can grow plenty of food on a steep hill side, you just have to terrace it and can't use a tractor.