The point is entirely the land clear rule.
Many of us recall the popular transit down the Lea valley over City and out the other side. The argument was there was always plenty of water based opportunities to ditch in the valley, the Thames and surprisingly City south is not too built up. If it goes silent up front and you ditch who knows what view the Courts might take. In yester year I have lost count of the number of times I did the run down the valley, in more recent times I have tended to only go that way in a twin - although I am not entirely sure why. Running up the Thames form the QE2 to City and back would not unduly worry me at all either with the appropriate clearance if City were open or other wise if not.
To be fair although the Lea valley is quite fun and interesting the navigation north is tight over the top of Panshanger leaving Stansted to the east, and I doubt it significantly reduces the distance compared with going the other side of Stansted, but each to their own.