Ignoring how accurate calling it RF is, so the experiment showed that electrons travel faster through air than through copper, you can see that, and the two delays are what you'd expect them to be?
The transmission speed in air would be very close to a vacuum, so the speed of light (c). This depends on the velocity factor [1], for pure copper it's 0.95 c, so 95% the speed of light.
Actually for air, according to this source [2], the velocity factor is 0.999707085823853610892 c.
Actually for air, according to this source [2], the velocity factor is 0.999707085823853610892 c.
[1] https://lowpowerlab.com/guide/rf-best-practices/velocity-fac...
[2] https://www.qsl.net/ac5jw/isbp/vf.html