RNS doesn't really map to the OSI model very well. It takes up basically every layer, 2-6, although it does depend on how it's used. The image on https://linuxinabit.codeberg.page/blog/reticulum/ quite near the top explains the difference between Reticulum and, as an example, TCP fairly well.

Your point about not having encryption is really quite interesting to me. I would want to add some form of signing to prove something came from somewhere unaltered, but that would go against the idea.