Fact, Bamboo must be heat cured in order to drive moisture out of it. Structurally, the lignin fibers within bamboo are so displaced between node fibers that it will suck up granules of sand. It is these gaps in the fibers that make it inherently non-water resistant and a pain in the butt to plane. It doesn't matter if you have a section that is 100% power fibers (this is the top 2-3 mm) or 100% pith (inside). The further in you go, the more gaps in the fibers, hence the more water retention. The action of heating bamboo causes these fibers to displace water/moisture and "tighten," making it stronger. It does nothing to keep it from taking the moisture back in. Heat treated cane will, over time, gain moisture back. There is nothing, and I mean nothing, inherently water resistant about bamboo. I've worked with Tonkin cane almost daily for 10 years. If you'd like to test the theory, I'll send you some to play with. Note, I never implied or stated it was waterproof...I'm only telling you it is not naturally resistant to water--regardless of what factual information you have. You can; however, make it nearly water proof by impregnating it with either Nelsonite, Deks Olje, or even tile sealer!
Don