Strawman Cleanup on Aisle 7!
I’ve yet to read or hear of vegans making such a claim.
Veganism has an impact on on the entire agricultural business model because “growing food to feed animals for slaughter to feed humans” is far, far less efficient than “growing food to feed humans.” Doing the latter will by definition result in a reduction in the number of animals, animal waste, emissions, freshwater usage and farmland needed to feed said animals that are no longer being raised for slaughter.
Veganism is not a panacea for big ag. People go vegan for a variety of reasons, one huge one being not wanting to cause the suffering and horrible death of sentient beings. Climate change improvement is another. Curing all the ills of big ag has never been a claim of veganism.