I think when they say that, they're talking about humid substrates and misting required for a glass tank (most of these care sheets seem to have been written before PVC cages were popular) .....an enclosure sealed like yours should only require paper substrate. Indented kraft paper is the bee's knees btw (just order a big roll of it on amazon)....or newspaper works well.
Personally, I prefer the look of aspen over paper but it's just so messy. I always find myself vacuuming. Kraft paper for me.