That’s about efficient use of land space, not related to GHG specifically other than tangentially regarding deforestation. Also elsewhere in this thread cattle was accused of being inefficient precisely because they sit in warehouses and eat cereals instead of grass. If cattle can roam pastures and eat grass, that’s an equivalent amount of cereals that did not need to be grown, farm machinery that did not need to run (on fossil fuels) to grow them, and a good amount of land possibly too hilly and rugged for any use otherwise put to productive human use through grazing.