The material called phenolic is basically immune to just about any chemical that would ever be used around a muzzle loader.
It is a tough plastic that uses imbedded cloth in it to give it a lot of strength.
What you say is mostly correct. As I have posted many times I attempted selling Delrin rods for a long time. I like them because of strength and they don't harm barrels. So, at one point I ordered a bunch of different 3/8" X 4' rods from my supplier made from every type of material available on this planet. I was particularly attracted to some of the phenolics because the coloring and material pattern made them resemble wood. I found none that passed muster in the lateral strength department. All except Delrin broke easily when attempting to use in a ml rifle bore. Costly experiment but I learned a lot.