It's not random noise. Look at the images in the paper. Horizontal lines, vertical lines, snakeskin patterns, Minecraft textures. Examples of miscellaneous surface patterns, in other words.

Back before deep learning, people used to make recognizers for features like that as a lower level of feature recognition. Now it's expected that features will be derived automatically from real imagery. This is kind of a return to that level.

A useful training set might be a big texture library used for game development or animation. Those are easily available.