I think the distribution of ride lengths might partially have something to do with the nature of cab hails in NYC vs. other cities. I travel a lot, and in most cities, uber is my go-to cab service, because I don't know the local cab system, I might have to call multiple companies, I can't street hail in a lot of places, etc.
However, in NYC, it's often faster to street hail a yellow cab for short trips than it is to wait for an uber. So here, I mostly use uber for long trips (airports, midtown to the financial district, brooklyn, etc.) or for time-sensitive situations.
Another thing to note in his section with self driving cars: the pool of cars would be fixed in that case, but the amount of uber drivers fluctuates throughout the day. Now, uber doesn't really care if drivers are 70% utilized or 50% utilized, as their revenue doesn't depend on that, but they do have an interest in keeping the drivers marginally satisfied
Also I don't have the numbers but at least in SF, taking an uber to the airport is much cheaper than taking a cab ($25 vs 50 from my office, last I checked), so I could imagine that something like that skews the distribution of ride lengths.