In particular, he states:
If rocks flung off the moon approach the L5 point, and are caught by a station there, the total momentum of the station will change. And the only known way to deal with that is to fling mass back off the station in the opposite direction. Then he reconsidered, and stated:
It doesn't have to be the same amount of mass, but the momentum (mass times velocity) has to be the same. If it isn't, you get an orbital change. 
One way or another, rocks flung off the moon have to be decelerated somehow at the L5 point, and the only way we know of to do that is with rocket engines or some equivalent. The theoretical best case for such a system in terms of propellant efficiency is a particle accelerator which fires mass at nearly the speed of light, but such a system will have a very low thrust. All known high-thrust engines are extremely inefficient in terms of propellant. 
There are actually two ways to change momentum. The other is to use gravity.Unfortunately, I believe he missed a number of salient points:
There are further possibilities if you assume that skyhooks are practical; if my old calculations were correct the distance from the Moon to the Earth-Moon L1 point is only about 2000 miles, and the gravitational loads are small enough to allow such a skyhook to be built with graphite (not nanotube) fiber or even sapphire whiskers. To get mass off Luna using one of those all you would need to do is haul it out some distance beyond L1 and let it go; since the net attraction past L1 is Earthward it might even be feasible to power the entire lift process with gravity, allowing each payload falling toward the construction facility to pull the next one up from the lunar surface. (Tide gets the regolith out!)
I don't mean to put Stephen down, but he hasn't put as much study into the issue as the extremely acute minds of the Space Studies Institute era. I stand in awe at some of their work, such as the papers presented at the AAS conference on orbital tethers (volume 61, if you can find it). When that kind of brainpower has gone before, it behooves one to be careful before pronouncing something impossible.
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