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> light bounces off the walls of the fibre optics cable

This is true of multimode fibre, but not single mode. Long haul communications cables are single mode.

https://en.wikipedia.org/wiki/Single-mode_optical_fiber

Even for multimode fiber it's a huge simplification of the actual physics. The actual cause of the speed difference, for both single and multimode fibers, is the index of refraction of glass.

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