I might be wrong about this, but I have the impression that flax and bamboo fabrics are pretty different kettles of fish.
- Flax is already sort of stringy and close to being a thread in the way it grows.
- Bamboo is more like a raw material: you extract a liquid or sludge which can be used to make artificial thread, but you need possibly quite unpleasant chemicals to actually do the process and make the thread.
So in my mind, flax is genuinely wonderful stuff & should have its praises sung more often, whereas "made from bamboo" sounds greener than it really is.
Perhaps someone more expert can say if that impression is basically correct!