Do you have a source to back up that claim? Australia has specific problems with cats, but apart from that, without evidence to the contrary, i don't see cats as any more problematic than other wildlife. There are more pressing issues to do with birds, mostly involving loss of hedgerows and tree habitats from 'development', and climate change.
>i don't see cats as any more problematic than other wildlife
The massive numbers of them are the problem. The only other pet as common is a dog and they don't hunt birds just for the sake of hunting like many cats do.
That sounds like a problem with outdoor cats. Why call out indoor/outdoor cats?
The only part of the complaint specific to indoor/outdoor cats is that they hunt even though a human will feed them. That's a complaint about the cats' morality, not a complaint about their effects.
It also assumes without justification that humans don't feed their outdoor cats.
Cats hunting is a problem regardless of whether or not there are fed. It has zero to do with their 'morality'.
The problem is any cats that wander around killing multiple birds. So many people have cats that even a small percentage of them doing this is super destructive to the local birds.
It's answered in the same post you just quoted. They hunt so many birds they are destructive to the environment and are driving some to extinction.