At first, ReCaptcha was used to transcribe books, and then in 2012 Google began to use it for street data. The GP is referring to its newest use, which is based on an image classification algorithm. Image search used to rely on nearby words on websites and in the URL, since computer vision was just a research technology.

Ironically, this new captcha has already been attacked using Google's own reverse image search: https://www.blackhat.com/docs/asia-16/materials/asia-16-Siva...