I see this repeated over and over, but I never see any hard data, only graphs. I was under the impression that we use the non eatable parts of crops (i.e. the leaves, discarded fruit parts, roots etc.) to feed livestock, and grassy, unarable hills are also included in this stat. So it all boils down to being extremely efficient (we can't eat the parts of the plants that we discard, and we also can't eat grass from the lands where we can't plant anything else, so we feed them to animals to transform them into more food, throwing them away would be a waste).

I haven't seen any info that clearly states "this land is exclusively used to plant x squared miles of grain that will be only fed to livestock".

Also, from your link, it is clear that livestock AND fisheries produce 31% of the agricultural greenhouse gasses, while crops produce 27%, so not that big a difference.