The simplicity of it is logic defying. It used to be that you had to find crosswalks or move puzzle pieces or type blurred letters and numbers, but NOW all the sudden I can just click a box and HEY!, I’m human?
That’s hardly the Turing Test I’d expected.
Apart from the mouse thing (which I’m skeptical about), cloudflare also correlates your traffic with other sites hosted on cloudflare. Bots typically don’t visit many sites, click around there, find another one, etc, whereas humans will have visited other sites, will be slower at clicking the button, will have left comments on some sites.