That is correct. Lean conditions mean lower fuel use, so lower hydrocarbon use (HC), so lower carbon monoxide, carbon dioxide (CO, CO2) output. But they also mean more "free oxygen" was leftover from the combustion process, and that free oxygen in high temperatures found in a combustion chamber will bond to the nitrogen that is also found in the intake air charge, so that becomes NOx. EGR is designed to feed in some oxygen-depleted air from the exhaust side into the air inlet side, so that there is less free oxygen left for NOx to form, in hopes that more of it gets used to actually burn the fuel instead.
In a perfect engine: HC (fuel) and N+O2 (air) combine to make CO, CO2, H20, and N. All the oxygen would be used to burn the fuel, and the nitrogen in the air, normally just an inert gas, passes right through.
But there is no such thing as a perfect engine. And since diesels, by design, naturally run VERY lean, far leaner than a gasoline engine can under the same circumstances, there is ALWAYS an issue with free oxygen molecules in the combustion chamber. Diesels do not run at a fixed lambda with regards to air/fuel mixture like a gasoline engine generally does. They are always under a varying mix. So it is hard to control NOx.
What sucks is, and nobody seems to want to point this out (the media certainly doesn't), is that despite the way higher than ideal NOx output of the engines in question, the other pollutants, HC, CO, CO2, etc., are far LOWER than the standards... in some cases, in the single percentiles. Which is why they get such great fuel economy. It also sucks that diesels seem to be forced into NOx standards the same as gasoline engines, yet gasoline engines are note expected to even come close to the CO emissions (fuel economy) that the diesels can get so easily. It is a double standard, and like it or not, that standard sucks. However, I do feel it is shady that any manufacturer, ESPECIALLY one of the largest on the planet AND one that has championed diesel technology more than any other, would "cheat" and do what they did.
So in conclusion of this post, yes it is most certainly possible than a 15 MPG pig of an SUV can emit far less NOx than a 50 MPG diesel. And if NOx is the ONLY data point you are looking at, then yes that 15 MPG pig can be "cleaner" and perfectly legit in the eyes of the EPA. I do not agree with this, since it seems to push for higher fuel use overall. It reminds me of low-flow toilets. Sure, they don't use as much water per flush, but if you have to flush them three times to make them do their job.... ????