It uses color, depth and subpixel motion vectors of 1-4 previous frames. All things that modern game engines can easily calculate. You didn't even need to read the paper to get this info, it's literally in a picture on the blog post.

The low-res image is itself output generated from a lot of other input the game engine generates. That same input that is already being generated anyway can also be fed into this to improve the post-processing. Finding ways to productively reuse already existing/generated data is the hallmark of any top (graphically) game.

I read a detailed write-up on the graphics pipeline for GTA:V on Xbox 360. It blew my mind how many different ways they reused every single bit that ever hit the RAM. Which explains how they pulled off those graphics on a system with half as much RAM as an Apple Watch.