It could be ML prediction based on scrolling + mouse movements or just old-school code which finds the closest and brightest div next to mouse coordinates. Maybe it's slightly more useful than plain mouse heat map which all competitors use, even if it's a crude approximation.

I assume they ran experiments with test subjects navigating websites with both eye and mouse movement tracking, and then trained a model to predict eye tracking from the mouse movement.

Wouldn't they have to train someone using your web site though. So I wonder what happens if you change your site.

I don't know if there is an actual working product though, their Twitter is only 16 hours old.