And how many do wild cats kill? And how many would wild cats kill, if we allowed them to multiply as they want?

The paper is about the species of "domestic cats", which includes feral cats, strays, etc, with unowned cats making up the majority of the kills as compared to owned cats/pets

Noth America had cats before the Europeans came, just not any in the niche that house cats fill. Think less barn cat more bob cat or mountain lion.