IANAS but from what I understand, the speed is constant, but it appears to travel more slowly due to the atoms found along the way, which absorb and reemit the photon.
In the case of the fibers, the light just bounces on the walls of the tube, effectively lengthening the distance it has to travel.
It's like saying that I did not drive a 50 mph simply because I took a longer detour: I still drove 50 mph, but I did not go in a straight line.