You missed a crucial bit of the abstract (my emphasis):
> We estimate that free-ranging domestic cats kill 1.4-3.7 billion birds and 6.9-20.7 billion mammals annually. Un-owned cats, as opposed to owned pets, cause the majority of this mortality.
From the text of the paper, "majority" means 69% for birds and 89% for mammals. Feral cats kill a lot of animals, because that's what they eat. Pet cats do not. "Domestic cat" here means F. catus, not cats that live in homes.
Also, i wouldn't attach complete confidence to this paper. Firstly, it was written by bird conservationists, who have a dog in this fight (as it were). Secondly, it contains some dubious numbers (eg bird return rate per owned cat per year of 4.2 - 18.3 in US and Europe studies - and yet my mum's cats all brought in 0 - 1 per year). I'm not saying ignore it, but don't treat it as gospel - and it would be great to see responses to it, and other work on this subject.
The last cat I had that was allowed to go outside was a fearsome mouser, delighted in toying with them and was quite the biology teacher, showing off their inner anatomy to us kids.
However, birds mercilessly teased him as he had no clue how to catch one, surrounding him while he lay on the grass, chirping, then flying just far enough away when he'd move towards them.
A neighbors cat was being harassed by a bird once. It would dive bomb the cat then fly away quickly. It did this multiple times while I watched. The cat got tired of it after a while and stood perfectly still. The bird swooped down toward it and, just as it was close enough, the cat leapt in the air twisting its body upside down mid leap and grabbed the bird from the air. It was the first time I saw how a normal cat could be such an effective natural killer.
Seagulls hang in the air at the Helsinki market square, opportunistically snatching french fries and ice cream and such from tourists. It has occurred to me that any person as focused as that cat could lunge and grab one of these winged vermin out of the air. I myself would not try it because I'd be afraid of losing an eye to a panicking bird with a quite long beak.
Not quite sure I understand the point you're trying to make, because getting back to the original question, though, "Are modern cats even that good at killing rats anymore?" - the answer is yes, absolutely.
There is no real genetic difference, as a total population, between feral cats and cats owned as pets. Absolutely, as you point out, owned cats who know they'll get fed inside will murder outdoor animals just for fun, and they don't need to eat them to survive - but that's a learned behavior, nothing they were bred for. You take a normal tabby and leave it to have its litter outside, unowned, and those cats will turn into feral cats.
Many animal rescue organizations have developed "Barn cat programs" for feral cats. These cats have not been socialized to be around humans so they make bad pets, but they're happy to live in a barn and ensure it says rat-free: https://www.austinpetsalive.org/programs/barn-cat-program.
> We estimate that free-ranging domestic cats kill 1.4-3.7 billion birds and 6.9-20.7 billion mammals annually. Un-owned cats, as opposed to owned pets, cause the majority of this mortality.
From the text of the paper, "majority" means 69% for birds and 89% for mammals. Feral cats kill a lot of animals, because that's what they eat. Pet cats do not. "Domestic cat" here means F. catus, not cats that live in homes.
Also, i wouldn't attach complete confidence to this paper. Firstly, it was written by bird conservationists, who have a dog in this fight (as it were). Secondly, it contains some dubious numbers (eg bird return rate per owned cat per year of 4.2 - 18.3 in US and Europe studies - and yet my mum's cats all brought in 0 - 1 per year). I'm not saying ignore it, but don't treat it as gospel - and it would be great to see responses to it, and other work on this subject.