I'm sure, cats and dogs owner will find thousand explanations why the ban of domestic dogs and cats on graveyards is not a relevant cause for the biodiversity.
How would you even keep cats away from a graveyard?
It's simply the distance, I suppose. Poaching in the backyard is much easier than going long distances and crossing streets.
A patch of nature like that will support a cat or two. You just don't see them, feral cats are really like ghosts.
I'm sure outdoor cats kill many birds and small rodents, but how are dogs to blame here? Most of them rarely, if ever, catch and kill something, and they generally don't get to hunt in their neighbourhoods unattended for hours on a daily basis. They also can't climb trees.