Sorry for the late reply here – we were busy over the last few days with #CTAConf!
We use Unbounce Convertables to build that overlay (and many others!). If you go into the Unbounce landing page builder there's a section on the left called Convertables. See image below:
Once you're in there you can build overlays for any page based on whatever behaviour you'd like (on click, on scroll, on exit, etc.).