Originally Posted by cyccommute

While the hubs may be different, it's the rim that carries the size designation. A 700C (622mm ISO) rim is the same size as a 29er rim. The tires for both are interchangeable. The wider 29er may not fit in a road frame but it's the same size of tire. Hub width has nothing to do with the rim size. There are plenty of 135mm OLD bicycles using 700C rims. Touring bikes, for example, have used the mountain bike width hub for a long time. Crossbikes and, I suspect, gravel bikes as well. Some of the gravel bikes may even use the "Boost" spacing or will in the near future.