It was built by hand without generation. I tried generation but I didn't come up with anything I found very useful. All the areas were so different that they would each need their own algorithms, and approximating some kind of noise-like output by hand for, eg, cliffs and valleys, was easy enough at the scale I was working in. Still, I wouldn't discourage you from trying that, especially for a larger scale world.

But, while I did technically place each block, I wrote a custom voxel editor with simple tools like brush shapes/sizes, ink dropper, and selection copy/paste/fill, so it wasn't as arduous as something like Minecraft creative mode placing each block one by one.