Bamboo is incredibly strong. In China they use it as scaffolding for building skyscapers. Here in Japan it is used for making some of the best flyfishing rods, kendo swords, and kyudo (archery) bows. And if you look at this post by Steven Evans, you can see that it can be used for very strong packs, too. For the Mariposa, inside the stay sleeves, the bamboo is plenty strong enough, I would think. Probably stronger than the carbon fiber stays when in tension.