The problem as I understand it is that while cattle are often raised in such places - grazing on open grass plains which could not support other crops - they are usually transferred to feedlots later in their life, where a significant portion of the nutrients used to support them come from corn, soy, etc, which do come from croplands.

So in our current system it does appear that cattle, even if they occupy some grassland space which could not be more efficiently used, do inevitably use resources which could be more efficiently used to feed humans directly.