that's BS. The thing that causes conjestion on motorways - the sort of conjestion where you are sat in a jam or slow moving traffic, for no apparent reason whatsoever, is peoples behaviours - primarily they travel too close to the car in front. This means they over-react to speed changes, hit their brakes too hard, causing the person to hit their brakes too hard, then accellarate away, which sets up a caterpillar motion of people braking harder and harder as you travel up the traffic flow until such time they brake to a halt.
If people could drive sensibly, leave a sensible amount of space between the car in front, then speed is not an issue and traffic flow would be nice and smooth and even.
Lowering the speed limit reduces this caterpillar effect, but doesn't stop it and is treating the symptom and not the cause. Granted, the cause is something that is difficult to police and tackle.
Of course it doesn't help that you have vehicles with different speed limits using the motorway - lorries limited to 56mph, towing vehicles limited to 60mph and normal cars traveling at a wide range of speeds. This just causes people to have to frequently adjust their speeds which just sets up the caterpillar effect.