I'll have to research that one. I've never heard of catalytic converters causing more NOx, and it doesn't even compute as to how that would happen in my brain. I've only heard of NOx happening in high heat and high pressure environments. I can't see a catalytic converter causing anywhere near heat or pressure that is produced by the actual combustion event, the event that is, as far as I understand, the culprit of NOx production. Three-way cats even reduce NOx.
But what do you mean by leaner? Normally leaner means more air and less fuel, not less air. Air is dependent upon RPM and throttle position. Since the RPM is geared to the wheels, it's the throttle position that counts here. As you lean out an engine it loses power, meaning you have to open up the throttle more to get the same amount of power in order to do the same amount of work (for an example, maintain 65mph down a freeway.)
Also less air leads to more particulates, not less. That or a less homogenous air/fuel charge, which basically creates the same thing, rich pockets of fuel in the combustion chamber, which is why diesels create more particulates in spite of being leaner running engines.
I also was under the impression that catalytic converters were adopted to reduce HC and CO emissions, not particulate emissions.
A particulate/NOx balance does make some sense. Richer, more particulate and less NOx. Leaner, less particulate and more NOx. More EGR, more particulate and less NOx. Although here we have an example of the oposite. The car that runs richer is producing more NOx.
Edit
My take on it is this. NOx is produced close to, but leaner than stoichiometric. In a modern gasoline engine the goal is to make everything as homogenous as possible and as close to stoichiometric as possible. Then the resulting NOx, CO and HC emissions are eliminated in the catalytic converter. Particulates are near zero in a modern gasoline engine, with or without the cat.
In a diesel you have one area that is very rich and another that is very lean in the same combustion chamber. Now you have it so rich it causes particulates. The overall lean mix does reduce CO and HC to near nothing, but ashes are harder to burn so you get particulates. There is an area where things are a bit leaner than stoichiometric, causing that nasty NOx. Then the rest is too lean to cause NOx, but still burns up HC and CO nicely.
In an older gasoline engine you don't get as good of a homogenous air fuel mix. You get cylinders and even pockets in each cylinder that are leaner and richer, which in turn produce larger amounts of NOx and HC and CO emissions respectively. The overall combustion isn't lean enough to eliminate the HC and CO like in a diesel either. And there really isn't any lean zone lean enough to stop NOx production like in a diesel. So you get way more NOx, HC and CO. This was the way it was back when diesels came out. They produced way less NOx, HC and CO than gasoline engines, with only particulates being higher.
But the NOx emissions are hard to reduce in a diesel. In a modern gasoline engine they are still being produced, but they are easily reduced with the HC emissions in a three way cat. The HC is used to strip the oxygen off the NOx and so you basically kill two birds with one stone. And whatever HC and CO emissions are left after reducing NOx these are burnt up in the second part of the catalytic converter with either the little oxygen left in the exhaust or with added air from a pump.
But a diesel always runs lean. There is no way to make a diesel run well at stoichiometric. So you get a lot of oxygen in the exhaust. With the extra oxygen any catalytic converter will not be able to use the HC emissions effectively to strip oxygen off the NOx molecules because it's too easy for them to just strip normal oxygen molecules from the exhaust. Also, a diesel produces less HC emissions from the engine in the first place, so again you have less to work with. So now you have NOx and nothing to lower them. Unless you squirt something like extra fuel into the exhaust. That's how VW got good emissions but poor fuel mileage at the lab under testing conditions. Or DEF.
But regardless, if you have to chose between a newer diesel and an older gasoline, the newer diesel will likely get the better emissions. So it's not just a diesel vs. gasoline. It's a newer diesel vs a newer gasoline.
Which ironically brings up a valid point. If older diesels are banned for NOx reasons, so should older gasoline vehicles that are also getting much worse NOx than modern vehicles.