It isn't!! The Kleenex is just as air-permeable as before. It's very very slightly stiffer, and I have a very bad feeling about what would happen if I tried to blow my nose in it, but this isn't like a rubberized coating. It's just a chemical that reacts with the electrostatic charge in water molecules in such a way that they don't like each other. It's like reverse surface tension.
I have a feeling that goretex is on its way out the door.