Diesels always run lean, they are not wasteful like petrol. Therefore if you restrict the air what can happen?
I think what you have been reading is not strictly true because we can have egr or exhaust treatment or both, the automotive side is still behind the commercial side.
Egr can increase emissions because we are sending exhaust gas which contains soot back into the intake and if not working correctly causes running problems, but I would not say they have caused the need for exhaust system treatment systems.