I heard the Tortoises eat cow poop. I think there may be circumstantial evidence of this in the article...
I would imagine it is because the cattle are grazing down the grassy fodder causes it to refresh itself more often, thereby increasing the amount of food for the tortoise. And since a significant amount of undigested fiber is in the cow poop your theory may also play a big part in the equation.
It would be interesting to see if there are any records of the number of bison grazing in that area. If there was a significant number that would be the predecessor to the cattle and your idea might be correct.
The hoofs of the cattle break the crusty sunbaked top layer of desert soil and leave depressions which become a collection point for blowing seeds. When it rains or there is sufficient moisture for heavy morning dew, the seeds hatch and new plant life starts. It is this plant life that is the diet of desert tortoises.
The cow manure is simply fertilizer for the plants and vegetation in the area.