Speed of light in fiberoptic cable is slower than c for a different reason. The light is in something close to vacuum, signals travel slower than c because the light doesn’t follow a straight path, it zig zags bouncing off the walls.

A radio wave or laser in reasonable vacuum (in orbit for example) will be lower latency than a signal on a fiber link the same length

I’m expecting lower ping via starlink than fiber once starlink has laser links between satellites