Well, technically, it is machine learning. Only that the machine learning was likely part of the usual data mining on google accounts and not much specific to the captcha problem...

(That said, whenever I used that checkbox widget they had before this announcement, there was a noticeable framerate drop in the browser while the thing was doing its magic. So I suspect, they are at least doing some browser fingerprinting/benchmarking to see if the widget runs inside selenium or a stock browser.

I also remember rumors that they analyze keyboard/mouse input on the page and check if it looks "human", but I'm not sure if that's true.)