The natural carrying population of bison eating grass is going to be considerably smaller than cows being fed grains produced using artificial fertilizer and major irrigation projects, given antibiotics to grow larger, etc. Part of what makes American beef so carbon intensive is that massive supply chain moving around animals and feed.
Another thing to consider is that American beef cattle are famously rich methane sources due to the artificial diet they’re fed and how early calves are started on corn. Not only are there fewer bison, they would be smaller and eating the food they evolved to eat so each of those animals would be smaller and less gassy.
