"Regular" pure glass fibers would be nearly as flexible. As an analogy - the same way that a solid steel rod would be strong, but if you make steel fibers that

"Regular" pure glass fibers would be nearly as flexible. As an analogy - the same way that a solid steel rod would be strong, but if you make steel fibers that are very thin they can easily be shaped and bent. The material properties did not not change, just the difference between having a "lot" of something vs very little. In this case, dopants are added to the glass to improve its properties as a fiber optic cable and the processing itself is important to attain certain properties. But even without those added dopants regular silicon dioxide glass would make flexible glass fibers.