The reverse arch does not necessitate having the large amount of exposed stanchion that is found on Manitou forks. The Travis for example, set at the lowest axle-to-crown height possible, has about an inch of clearance between lower crown and casting at bottom out. If the stanchions didn't taper to a larger diameter at the lower crown, I could lower the crown well over half that distance before the arch would be anywhere close to contacting the downtube. And my bike has a larger downtube gusset than most. There is also no reason why Manitou couldn't have raised the casting up. The distance between the seals and the top of the arch on a Manitou is much longer than on say, a Marzocchi.It requires a longer fork and more exposed stanchion so the reverse arch doesn't hit the downtube.
Kind of self defeating.