The idea is, they don't eat meat and don't use leather. Combined, they make a whole cow that wasn't slaughtered.
Yes, but that's some kind of morale position. It's not an environmental position. Once the cow is killed for meat, using the hide for leather is more environmentally-friendly than throwing it away. And being natural, leather will decompose nicely when it's beyond its useful life.
I can see being against eating meat as some kind of environmental stance, as cows consume a lot of resources. But it's the meat-eating that drives the livestock industry, not the skins-- they are just a byproduct. Not making use of the skins is just wasteful, it seems to me.