hmm i think you actually can use other 1 1/4 stems too
Aero gravel might be a thing now, but I think taking track bike geometry to the world of gravé might be a step too far. The non-standard 1 ¼ steerer also massively reduces the number of stems available.
First use I saw of it was on a Rapha Hell of the North a few years ago, the native French speaker I was very grumpy about how stupid is was as the French for gravel is gravier
hmm i think you actually can use other 1 1/4 stems too
http://www.bikeradar.com/road/gear/article/canyon-cp01-hoverbar-specs-details-51934/