I am also surprised at how far the skateboarders can go, but to be fair your estimate of 5-10x is way too large.

The cyclist in this case averaged 37.5 km/h, which is really impressive over 24 hours. 5-10x slower would be 3.75-7.5 km/h, so somewhere between a very slow walk and a very, very slow jog.

Plus, the skateboards they are using are not going to be regular ones but purpose built and similar to downhill longboards.