They used a very simple neural network to do the detection. Several example pictures were included in the write up, showing the cat normally, one where it was holding a mouse, as well as a profile of a possum (IIRC).
Replicating those results has been on my to-do list for over a decade.
You can piggy back on my object detection api - it can detect birds - just tried with the image => https://images.unsplash.com/photo-1520638023360-6def43369781...
Request example => https://pastebin.com/1Wnyb6PD
