That's not true. The typical total reflection angle in single mode fiber optic cable is less than 4 degrees. The refractice index of glass, however, is about 1.45. So almost all the slowdown is due to the glass directly slowing down the signal.

Though you can still get a group delay (slower than free space) in vacuum filled RF waveguide. When you break down the single mode, it appears to be zig-zagging through the waveguide (bouncing off the walls), and slows down as cutoff is approached. Like in fiber, the modal dispersion is separate from the material dispersion.

AT&T was going to build out a cross country, single mode, circular waveguide network in the 60’s, but was obsoleted due to fiber. Maybe that’s what the HFT ought to due, or maybe air-line coax.

I think the distance is less of an issue than the temporal smear that the bouncing causes. A clean square pulse turns into a blob.

Temporal smear is caused by the varying refeactive index of glass with wavelength and the fact that a short time pulse must have finite (large) spectral width (by the uncertainty principle), not by the bouncing.

So, the different path lengths implied by internal reflections don't contribute anything to the smear?

By the way, the uncertainty principle doesn't say we can't have a narrow pulse. Frequency versus time can be used as an analogy to help explain the uncertainty principle. If we know a precise frequency, the signal is poorly localized in the time domain. If a signal is narrowly localized in the time domain, it is poorly localized in the frequency domain. The uncertainty principle is something else, though: it revolves around not being able to make certain related measurements simultaneously, at the quantum level. We can be quite certain about the frequency content of a transient pulse and its exact temporal shape, if it's not something from the quantum domain.