When I was younger, I was really worried about learning how to imagine, specifically in regards to music. So many musicians seemed to have started out at ridiculously young ages, and despite the fact that I had been plunking away at piano lessons for many years, I still couldn't hear any original music in my head. Then, when I got to college, I decided to do a side-major in music, just for fun. As I was doing my listening exercises (hours of analytical listening) and diving into the depths of music theory, I started noticing something in my head opening up. I could hear snippets of music... recombined from others I had heard recently, perhaps, but with definite original streaks to them. I was not actively playing an instrument at the time. I was not even composing. All I was doing was building a solid musical foundation. And just like that, after a couple of "aha!" moments, music was no longer a formless mist. It had structure. It had rules (that you could elect to break). Now that I could see the walls of my room, I could finally start to build on my own.

Anyway, my hunch (and reading between the lines of the article) is that something similar happens with art. Once you learn to intuitively visualize shapes in three dimensions, and once you develop a large mental "reference library" of architecture/landscape/characters/etc., you'll be able to create imaginitive new worlds without much effort. But without having that foundation, you're still stumbling around in the dark.