That’s only one metric to compare. That doesn’t take into account the carbon sequestration from manure, topsoil formation, etc. from grass fed animals.

It also doesn’t take into account the massive energy use (and therefore, carbon release) of fertilizing, seeding, irrigating, harvesting and processing feed for conventional cattle.

So in total, I would expect grass fed meat to be much better than conventional meat, and possibly better than if we were to just eat crops directly.

In North America, much of the land now used for grazing cows was formerly occupied by bison (which are so close to cows that they can actually interbred).

I seriously doubt that this caused any significant net change in methane production.