That study's been going around for years in the media, but mainly because it's sensational. If you actually read the article, I'd hardly say it's very convincing, or very accurate. Also, this.

Existing estimates of mortality from cat predation are speculative and not based on scientific data13,14,15,16 or, at best, are based on extrapolation of results from a single study18. In addition, no large-scale mortality estimates exist for mammals, which form a substantial component of cat diets.

"About a third of the problem" So, not the primary cause (or solution.)

https://doi.org/10.1038/ncomms2380

"We estimate that free-ranging domestic cats kill 1.3–4.0 billion birds and 6.3–22.3 billion mammals annually. Un-owned cats, as opposed to owned pets, cause the majority of this mortality."

The article also states the following regarding more popular studies in the media involving pet cats: "The magnitude of mortality they cause in mainland areas remains speculative, with large-scale estimates based on non-systematic analyses and little consideration of scientific data"

Indoor cats are not a problem because they are indoors. Outdoor cats thar come inside sometimes are a problem indeed but most of them were not adopted they just apeared out of nowhere and you now think it's your cat. So it was feral at some point or at least was born from one.