Hi,
that slider is also in demo content, it is called Collections, you can import it and it should work right away and you will be able to see how we did it :)
Here is a short explanation: we have used Evora-overlay style (it will be available when you import slider) on image layer, and on video layer, we have used a trick – another layer over the video with transparent png image and also Evora-overlay style applied on it..
Cheers