I'd suspect the latency improvement is control over the entire path and not specifically about the propagation speed/delay.
Getting your data across a long haul undersea fiber requires aggregating it with everyone else's data at a central point before it gets sent out optically. This adds latency.
Even with the propagation variability of the wireless path you might be saving hundreds of nanoseconds.
There is some latency savings because the radio path is far straighter than the fiber path, even considering the ricochetting off the ionosphere and the earth. But the bulk of the latency savings comes from the fact that radio waves move at the full speed of light while photons through fiber only move at about 2/3 of that speed. So yeah, it works out to about a 10 ms savings on shortwave compared to fiber.
I doubt that this is a major factor. The extra delay (light speed vs. fibre speed) for signals across the Atlantic is at least 10ms each way, a few hundred nanoseconds for aggregation shouldn't make a difference there.
Getting your data across a long haul undersea fiber requires aggregating it with everyone else's data at a central point before it gets sent out optically. This adds latency.
Even with the propagation variability of the wireless path you might be saving hundreds of nanoseconds.