wow, that video is from monte blanco, veracruz, where some of the bamboo for my bike came from!
i am going there next month to source some more bamboo and observe their techniques.
i think thinner culms, such as for chainstays, are easier to bend, prolly no sand needed. a friend also had an idea of training the bamboo to grow into the shapes you need, you could potentially grow a whole bike, even insert the bb and headtube at the right times and have the bamboo envelop them, growing the bamboo into a jig! you would have to wait 4 years for your bike, but it would be pretty wild!