I own a Penny Farthing, but not near the quality of these bikes.
I have pedalled mine at speeds very near 20mph. It has the same problem as a unicycle at high speeds: The wheel wobbles twice per rotation because of the weight of the crank arms.
If you watch the tire ahead of the contact point on the road, you will see the tire seperate from the rim, several times, because it is loose. Rubber stretches with time, even though the wire in the center is tryig to hold it tight to the rim. High speed, the wobble and the loose rubber allow the tire to spin off the rim. The rider actually controls it for a long time with no rubber. I think he puts his legs over the bars to help control the wobble. Plus, where else would he put them -you can not spin that fast.
I watched John Foss track-stand mine about as long as the other guy, but he didn’t get it going backwards. He’s got a real quality bike there. All frame tubes are doubled and the hubs and bearings are huge. Idling on a Penny Farthing is nothing like idling on a unicycle. I can idle all day on a coker or regular uni (one footed), but cannot idle at all on my PF. I can track-stand for a little while, but I’ll have to try the half-cocked position this guy uses.